Nothing has 'happened' yet. There has been no 'incident'. Posters telling you otherwise are incorrect.
All that has happened so far is that you have had a verbal conversation with your HT who has made some vague off the record threats about capability. You have had none of this communicated to you in writing and no official procedures have been instigated against you. This chat is just that - a chat. It may go nowhere, it may go somewhere. But it's not an 'incident' and nothing has happened so far as the union would be concerned.
As such, if your HT were to commence capability procedures against you in September, then the union absolutely would be able to represent you now that you're a member. Don't worry about that.
I'm delighted that you're leaving as this school sounds horrifically toxic.
From September, make sure every email correspondence with your HT or any other SLT member that is disparaging or critical in any way is forwarded to your personal email so that you have a safe record.
Any 'chats' with the HT - make sure you have another member of staff with you. Refuse to continue the conversation if this request is denied and leave the room.
Follow up any conversation with the HT with an email, bcc'd to your personal account, summarising what was discussed and agreed going forward, so that there is a written record of what has been said to you.
Make sure you have records of everything you have communicated about the TA and everything you have done to report the situation. Any emails you have sent, again, forward to your personal account. Create a dossier of evidence supporting you, just in case you need it.
At the end of the day, as others have said, capability is a long process and it will be utterly pointless to instigate capability procedures against someone who is already leaving in January. However, the HT may want to do this to make it look like she is doing something to address the Ofsted report. You could become her scapegoat and she could make your life very unpleasant if she chose to. So you need to make it just as difficult for her in return. Refusing to meet with her, ever, for anything, unless you have advance written notice informing you of what the subject of the meeting will be, and a union rep with you. Minuting every meeting and sending these minutes to her with union rep cc'd. Asking her for evidence of every claim she is making against you, and so on and so forth. Fight fire with fire. She'll soon give up.