Wow Hon!
I just want to say how you are handeling this sounds very mature! I have never been in a situation like yours before (I'm not adopted), but having several adopted friends and famil7y members, and talking to them on this issue and others like this over the years, I can relate to where you are coming from on a friend stand-point.
Utimately, I say the dicision on what to do is yours and yours alone!
That said, you are obviously not out to harm your bmom (you sound soo sincere in protecting her feelings). You have honored her wishes not to yet meet her to this point, so, as long as you keep her feelings in mind and are reasonable, you have EVERY right in the world to meet your bgrandma (as you had every right to meet your baunt and any other family member that knows of you already and wishes to meet you) if that is her (your bgrandma's) wish. Think of it more as a mutual agreement between the two of you and not going behind your bmom because she has the opportunity to meet you too (and has declined) and your bgrandma was aware or told by your bmom of you, so you are not breaking an ethical boundary!
This is just me, but I think If I were in your shoes, I would need the closure of meeting her in my life, either when she is with us on this earth (to close the void of something missing for both of you) or, secondly, to show honor to her at her funeral and for closure. I feel if I were in your shoes, I would regret it if I did not make the trip. For me personally, I did not see my grandfather off to heaven 10 years ago because I was too imature and still to this day regret it!
Also, if she is stable, I see no reason why you can't and shouldn't go meet her ASAP and get to enjoy some time with her! She would prob. love it and it may AID in her recovery, not hinder it--something for her to look forward to! Talk to her or her drs., or your aunt about it and see if she/they show interest! No one else around has to know you are her granddaughter if you want to keep it under wraps for your mom's sake (if someone asks, you can just say long lost friends/associates that met via mutual contacts-and you wouldn't be lying).
So, to sum:
?1) I say do what you think is best for you and your bgrandma: consider if you or she will you regret it if you can't meet while she is in a good enough state to enjoy your meeting or, at least before she passes?
?2) YES, I say it's OK! Follow your desire with confidence!
?3) YES, I say it's OK! Follow your desire with confidence!
I will pray for you!
T.