For one, does STR have a sizable impact (if any impact at all) on Evie's smashes or normal attacks?
What Evie moves rely on ATTK and what moves rely on MATTK?
It does impact her damage, but not as much as INT. You'd still notice it, but STR isn't really a priority. As for the latter part of that, check the damage percentile guide.
It says AGI boosts the Critical chance of bolt magic, is that only limited to firebolt and magic arrow or does that include finger snap skills?
Snaps aren't bolts. About all you'd really get out of it is some noticeable extra stamina regen.
Does INT have diminishing returns on MATTK like STR has on ATTK?
Probably.
Which is better for bosses, bloody thread or death label?
Neither. The only bosses you'd really care about it on, you don't have time to do that big of a combo
and pull the soul. That could probably be debated... but generally smashes alone tend to be used on bosses, without pulling souls. Then, you blink away so you don't get raped. As for what's "better..." Well, Bloody Thread is level 40, so none of us can really say unless we played in Korea. Mark of Death, eh... it's alright damage. I generally just do a LLLLR instead though, seems faster.
Is there any growth of Mana shield's power, meaning will it be able to block the same amount of attacks from the bosses in Boat 3 and in Boat 5.
As far as I've seen, it never gets any stronger. But, I'm pretty sure one hit can only drop your shields and not your hp too. You can see that if you go stand under the spike trap, usually it's instant death but not if you have shields up.
Is is best for Evie to wear full cloth armor? I keep reading about people with 5/6 Blood Prince sets and reading that I should rank weight+heavy mastery.
That's... user preference. It really depends on your playstyle and what you want out of the character.
Some people prefer higher damage, others prefer higher survivability and armor that doesn't break in 1 hit.
Do whatever you feel is best for you.
Later on though, most of the good looking sets that I've seen tend to be plate or cloth, and you won't be able to wear the plate. So, if you care about looks then take a look at some of the sets available later on and see what you'd want to wear.