No, he wasn't. Briony accused him of attacking Lola out of a confused mix of emotions where she only surmised it was him. She was also angry with him for ignoring her, and jealous that he was attracted and involved with Cecilia. She had also seen the 'note' that Robbie had written, which shocked her, but also fascinated her and made her more jealous. Briony shared the contents of this note to Lola as a child would do to impress Lola with her knowledge and experience. Lola and Briony excitedly convinced each other he must be a sex maniac.
As Briony was angry with him and both mystified and confused by the sexual affair between Robbie and Cecilia, she could have allowed herself to jump to conclusions over something she had not really seen and accuse him. Whether she knew she was guessing at the time is unclear, but she seems sincere.
There is much more ambiguity in the novel than in the film. In the novel Briony has no light. It is utterly dark and she can only sense that a shape is with Lola (and the shape quickly takes off when Briony comes close), and hear Lola crying. So in the film she clearly sees Marshall's face (although apparently represses the memory). In the novel it could have been either Robbie or Marshall: there was simply no way for Briony to know for sure. Because of this, Briony's process of coming in the novel to understand that it was Marshall and not Robbie is far more round-about and indirect.
The police and family believed Briony as Robbie was of lower class, and Briony swore on what she had seen. That Briony was able to produce the written explicit note from Robbie to Cecilia gave her charges an added, though unrelated, sense of confirmation. The police would have seen this as at least worthy of making the arrest. It would seem that the twins or Cecilia might have been able to confirm his whereabouts during the search.
Briony twisted her mind to believe that she saw Robbie do it, but when she sees Marshall at Marshall and Lola's wedding, she realizes that it was Marshall who was with Lola, and it is far too late. Regardless, Robbie was of low class, and Marshall was of high class, therefore the police would have been predisposed to suspect Robbie.
There is ambiguous evidence in the film that the liaison between Lola and Marshall could have been either an assault or a willing assignation or a seduction, whether craven or consensual or some mixture of innocent flirtation that was then abused. Those who have read the novel claim the book makes it slightly more explicit that this was a consensual act.
Based only on the film, the morality of this interaction given Lola's age (15) is clearly problematic and likely to be statutory rape (though not sure of British laws of 1935), but Lola (and the twins) have previously shown they know Marshall was a millionaire and Lola has been fretting over the financial instability of her divorced family. Lola was clearly interested in Marshall and flirting with him in response to his attention.
As Lola was underage, Marshall had to flee when discovered by Briony. Lola's initial response to Briony is "I'm sorry, I'm sorry," and she seems more embarrassed and startled by Briony than hurt. The film depicts the scene in a very ambiguous manner that leans towards rape, but the social context of the rest of the film certainly leans towards a consensual affair. At the moment of discovery, Marshall did have his hand over Lola's face - whether from trying to prevent her from crying out during an assault or from trying to cover the sounds of their lovemaking from an approaching flashlight is unclear.
Briony's interpretation is that she has witnessed a rape by an unknown assailant, and this makes a convenient cover for Lola (if she is seeking to protect Marshall). Lola never claims she was assaulted. The convenience of Briony's interpretation, particularly once Briony has confirmed that she thinks it might be Robbie, makes it a useful foil to protect Marshall if that is Lola's wish (further protecting Marshall, who would be without an alibi). Lola never goes so far as to identify anyone, or to falsely accuse someone she knows to be innocent, but she does go along with Briony's claim it as an assault by someone she could not identify. If it was an assignation arranged with Marshall, she does not move to protect the working-class Robbie. It is also deliberately left as a possibility that Lola was attacked, and never knew her attacker.
Aside from the odd dialog with Briony at the moment of discovery where Lola is falling all over herself to apologize, there are prior suggestions that Marshall and Lola had already had sexual interactions. Marshall shows early and inappropriate interest in her, the kids and her trousers, and rather lewdly seduces her with the candy bar in a heavily sexualized scene.
They come to dinner together in apparent good spirits with each other and sit beside each other, she arriving with rope burns around her wrists. Marshall quickly interjects, before she can come up with a response when Briony mentions the marks, blaming the fresh marks on an "Indian burn" from the long-departed twins. (The visual indentation do not look like an Indian burn, but seem to be clearly the marks from her being tied up with a rope). Lola generally, but not specifically, supports this story at the dinner table. Emily (the mother) finds this unlikely behavior by the twins and directly asks Lola if indeed the twins did this. Lola does not reply and Marshall quickly interrupts to confirm this, and also explains away an injury on his forehead as coming from rough-housing with Lola and the twins.
Lola refers to this roughhousing earlier with Briony and speaks of being bruised and made black and blue by the twins and that she is having a terrible day. This sounds false and theatrical when presented to Briony, in part because the earlier scene had shown Lola to be entirely in command of the very compliant twins. It seems more likely to be an arranged explanation for the physical evidence of Marshall's and Lola's rough play, concocted jointly by them. Lola is histrionic in describing it to Briony and then drops the issue and the drama entirely when Briony changes the subject.
The twins refer in their note that they were leaving because Lola was mean to them, likely shunting them off so she could be with Marshall. Some sexual interaction between Lola and Marshall seems heavily indicated.
There is nothing in Lola's behavior at dinner to suggest she was uncomfortable with Marshall. The twins have not come to dinner so Briony is sent to fetch them and it is then discovered that they are long gone. Something that entirely startles Lola, who was supposedly minding them all day.
The clearest confirmation that the union of Marshall and Lola was a mutually agreed upon, or at least accepted, tryst comes from the fact that as soon as she became old enough, she accepted Marshall's offer of marriage. There are again multiple interpretations here. Lola could not have known in was Marshall, Marshall could be protecting himself from prosecution, or Lola could have been traumatized by the assault into identifying with Marshall. The marriage scene, as played, certainly suggested a consensual reunion of the two lovers.
Briony's shock on learning of their marriage is what immediately causes her to realize the identity of the man in the second scene of adult sexuality that she may have tragically misinterpreted. Their shock and surprise and shame at seeing Briony at the wedding spoke volumes, as they too now knew she must be aware of their original scene and thus their complicity in Robbie's false imprisonment. Further, the director heavily underscores their guilt and Briony's agency by choosing to highlight Briony witnessing the pastor's calling for anyone to testify if this union was improper.
As usual, in real life, Briony cannot come forward to tell the truth. She can only try to set things right in her fiction. There are many curious parallels here. Briony is too young to imagine sexual desire and still has a romantic and chaste interpretation of sexual love. Also, her dramatic narrative skills cause her to reshape the ambiguous nature of two sexual encounters, and in both cases she cannot conceive that they were consensual. There is a balance here in showing the tragic effects of her not understanding this.
On the other side, one could argue that the film is just as effective in formal balance to have the pure love of Cecelia and Robbie remain unfulfilled, but to have the improper and squalid love of Lola and Marshall be recognized in a formal wedding. This could be seen as a balancing point whether it is a holy blessing of something that started as rape or as a rather ugly or mercenary statutory debauch.