I think you are conflating a number of issues, OP.
IF it is correct that the teacher is trying to force your DD to play only with Daisy, to the exclusion of her other friends, then you would be entirely within your rights to ask that it is handled differently. Perhaps more than one child could be asked to be Daisy's "buddy", for example.
But I very much doubt that is what is happening. It sounds like the teacher might think that there is a risk that your daughter and her best friend are becoming isolated, if they only want to play together and won't join in with group games, and is trying to kill two birds with one stone - Daisy is included, and your daughter and the other girl widen their social horizons a little.
However, it sounds as though your particular issue with this situation isn't the disruption to your daughter's chosen friendship, but the fact that Daisy has "issues", and isn't one of the bright children who you would prefer your daughter to be spending time with. I would strongly suggest that you don't let even a hint of that attitude leak through into any dealings you do have with the teacher, as it will entirely negate any valid point you might have about the situation with the "forced" friendship.
I don't even know where to begin about the gossiping TA. Her behaviour is appalling. And yours isn't particularly great in that you've clearly listened and formed views based on that gossip. If a TA started trying to share confidential, sensitive info about a child at DS1's school, I would tell her I don't want to be privy to that sort of information and, if necessary, I would simply walk away. Confidentiality isn't optional - this is a massive, and potentially damaging, breach of trust.
The academic issues are something entirely separate. If this becomes a real concern - and I'd suggest you give it longer than a week - then you need to raise it as a specific issue, and not package it up as part of a general parcel of discontent with the overall situation.
To be honest, it sounds like you are just unhappy with your bright daughter, for whom you understandably want the best, being in a "boy-heavy" class, with a less experienced teacher and being paired up with a child you don't really want as a friend for her.
If that's the way you feel about it, then that's the way you feel. But I suggest you don't let your daughter know you think she's justified in finding Daisy a nuisance - it won't harm her to spend a few days putting someone else first - and I very strongly suggest you think hard about what parts of your concerns, if any, are legitimate mismanagement on the part of the teacher/school.