I am not, nor have I ever been an actor, so I may be all wet here. Given that it is one of Broadway's legendary roles, I have always felt that the role of Dolly was not actually that large or difficult. The role is all about charisma, wit and warmth, and who has more of that combination than Bette? (I remember concluding this the last time I saw Dolly with Carol Channing. She was 73 or 74 and I remembered wondering how she could get through it night after night. With that fear, I watched the performance and realized (a) that she was just as good as the first time I had seen it 30 years earlier and (b) that she was off-stage for long periods of time, which I assumed gave her a chance to rest at regular intervals.
Since there are good stretches of time when Dolly is not on stage, and -- other than the title number -- the dancing is of the (limited) soft shoe variety, I am sure that Bette will be fine, and we will all love her performance.
PS -- I have to admit that I am actually looking more forward to Donna Murphy's performance for the weirdest reason. I already feel that I can visualize Bette's performance, with the wide grins and great warmth, and will be very surprised if it is not what I expect. On the other hand, I have no pre-conception as to how Murphy will be, e.g., how will she play the broader scenes? Will she play it straighter and emphasize the pathos? Etc.
I have always see her as a subtle / dramatic performer, although I remember reading how outrageously funny she was in (Song of Singapore ?????) years ago. So, I am really anxious to see how she does.