Is it possible to start a romance with Tali if you used the evidence during her trial?
Yes I believe it is possible to start the romance, yet it will be a rocky start
It just seems odd to use the evidence against her and then romance her imo.
It was an accident. Sortl of.
Does it gain her loyalty?
No, the only way you can gain her loyalty is to not show evidence, have Tal'Regar live and let Veetor go with Tali, then if you talk to them BEFORE going to the Alari, an option should pop up saying 'incite the crowd', dropping the treason charge.
Tali is unlike Jack/Miranda dispute where you can side with one of them, but regain loyalty later. With Tali:Treason you can't regain her loyalty if you presented evidence to the Admiralty Board.
You can only romance someone who is loyal to you, otherwise the particulary dialog option is not available.
You can have her exiled, lie to the Board or rally the crowd, like XXVenomsnakexX0 wrote. They all lead to her become loyal and as such romanceable.
Presenting the Evidence however doesn't only make her mad, she will not become loyal and there for no Romance Plot is available with her, even if you side with her during the Tali/Legion dispute.
Restart mission or reload savegame.
Just a comment: in one playthrough I left Kal'Reagar die (not on purpose) and yet I got Tali's loyalty (but, of course, didn't give the evidence to the Admiralty board!).
What do you think?