If she still doesn't know if she can attend the wedding (which granted is months away) then I would advise not giving her a "task" at the wedding, since as you say that is likely to pressure her to attend despite hardship or decline while feeling guilty. That includes witness, singing, doing a reading, lighting candles, bridesmaid, etc.. If at some point over the summer she's able to definitely say yes, she'll be there, great! Maybe you can find a special way for her to participate then, if you want--not being a witness, but something else meaningful.
My friend Amy wanted me to be a bridesmaid in her wedding, but I wasn't interested in doing the bridesmaid thing for anyone. So she asked me to do a couple of readings instead. Turned out I was the only person doing them, and it was actually one of the most visible roles in the wedding--I think the only person who got up in front of people and said more stuff was the minister! I don't know what other roles might be available in your situation, but they definitely don't have to be "lesser" in sentiment--just "lesser" in legal necessity.
If it comes up in general wedding conversation, I would go ahead and mention who you've chosen (like if it would be weird to NOT say), but not tell her about the dilemma you faced over her.