Perhaps. I would expect the magus bringing it into combat to have some skill at doing so, otherwise they'd choose another option. Let's figure at least an Ability with specialty of 5 - not overly high. If it's enchanted, it's at least Superior if not Excellent (only needs to be +1), and if it's going to be a Talisman it will probably be Excellent. So let's figure we're at 8 now. 7 magnitudes of effects would be needed. That's a fair amount for a typical item, but for a Talisman you'd probably get to it pretty quickly since you get other bonuses while doing so.
Right and wrong. The duration can't be Momentary and lasting via vis. Just use a Constant duration, which isn't really limited.