I highly doubt it that he becomes stronger because he will be adult
There are some reasons:
- He has no Fighting experience what so ever
- His training is mostly swinging a wooden sword against the air or unmoving objects
- He can only use CoO (stated in the Vivre Cards)
- He has never really trained his Artifical DF
- His mental age would be still that of a 8 yo
Yes. This is the biggest thing to me. Fine, physically someone could show up and age Momo, but at the end of the day, he won't get the 20 years of life experience. He's still 8!
I don't think it'll be Bonney though - last we saw, she was at Reverie, and God only knows what happened to her there. If she managed to escape, she's likely on the run and nowhere near Wano.
Shinobu can't - this is her Devil Fruit according to the Wikia:
The Juku Juku no Mi is a Paramecia-type Devil Fruit which allows the user to
mature and decay inanimate objects.