Some time ago here was explained why gen. pl. of неделя (недель+а)becomes недель applying the general rule for fem. and neuter.
A question close to this. In дат. / предлж. падеж мн. чс.: -ам, -ами.
For the same reasons it is clear why рубль -> рублям, здание ->зданям, and even ночь -> ночам because here of soft condition of ч before a. No problem. But why музей -> музеям, if no phonetical change for "e" would occur if it should be written музеам ? Perhaps there is no "because..." .
Examining the declension of край, края краю,... I see that it is "like" if й contains two ии in this cases. And we should have the second meaning of "soft" vowels, like not including a ь but two independent sounds "ia, ie, iy". I don't know. Thanks.