Sorry, I've never had the problem of Jaheira being unable to shapechange. But I find that the alternative forms aren't that great either, so I seldom used them in the first place.
If you dump Yoshimo, he won't attack later in the game.
You already have roughly 2 priests in the party (Anomen + Jaheira~0.5 + Aerie~0.5). Viconia is a better cleric in terms of additional spells, but I guess she is not as good in hand-to-hand combat as Anomen (since he was a fighter earlier). Depending on your preferences, it's better choose between these 2. Aerie is good because of her numerous spells in general, and Jaheira is great for druid spells (some are different from clerics).
The silver hilt and silver blade are obtained in later chapters (4-6) in the game. You should come across them naturally as you progress deeper into the plot, one of them when you finally return to Athklata.
If you need any more clarifications, pls feel free to ask
