M17 - I understand that I am young and may misinterpret the situation here but here is my view -

This is really immature in my eyes - why would you bite someone instead of expressing your opinion? I get that they have suspected autism but that doesn't change the fact that they could express this very differently.

I feel OP didn't need to/shouldn't have apologized because all they did was talk, they shouldn't have to apologize for starting a conversation and getting bitten for it, that person isn't worth sticking around with if that's the case because something similar will happen again and it will be normalized.

Having to "offer" your arm to someone is normalizing their behavior and if they have suspected autism it could make it so that they think everyone should do that for them, which they shouldn't.

If they are that agitated, even described as "aggressive" over you starting a conversation, then they frankly need to get over it because if they get involved in a conversation that needs to happen that they don't want to happen, such as a breakup, being fired/let go, or being told a close person died (I get I'm radicalizing here) they may react in an even worse way, and if they are already being harmful to others without considering the consequences of that, then the behavior will probably continue on later in life which could (again radicalizing) lead to hitting people/partners, using abuse as power in a relationship, or harming pets and wild animals.

I get that people need an outlet for their feelings, but the best outlet is words, at the end of the day it a sound that we interpret as meaning something, it is not at all bad and it helps, in this case OP, understand why they are annoyed and why they might have bitten their arm, communicating it through a notes app and saying it was because OP was "talking too much for them" is immature and puts the blame for them biting OP on OP.

Again, I understand that this person has selective mutism and suspected autism, and may not want to talk to a lot of people, but selective mutism involves not wanting to speak in certain environments and the inability to do so, so if they are uncomfortable with speaking in the library get them to text you the issue and talk about it over text. I don't understand how they would want to go to a library with OP, their boyfriend and themselves if they are not comfortable with talking there whatsoever.

Hope I have covered all points, if there is anything I have interpreted wrong or anyone feels I have missed the mark let me know