For your other question: yes, Multi-Attack will only use Silvally's attack stat in damage calculation.
Silvally is a Pokemon to use more as a pivot as it has access to Parting Shot, so should have maximum speed EVs and a speed boosting nature. If want Silvally to be a physical attacker, put some EVs in attack, and likewise if Silvally is a special attacker put vs in special attack; it has many great moves from both sides. I will say however, if you have Attack EVs, don't have any Special Attack EVs, and vice versa.
The rest of the EVs can be put in HP. This could be 4 EVs or hundreds, depending on how bulky you want Silvally to be, and what moves from others' you want it to live.
I sometimes use Silvally myself, and my EV spread is 252 HP / 4 SP. Atk / 252 SPD so is as bulky and as fast a pivot as possible, so can use as many parting shots as possible. It also means I can catch my opponent off-guard when I use both physical and special attacks against them with equal power, as I had no investment in either stat so there was nothing to lose.
Hope this helped!