General

100% crit from attacking a few times, or waiting for like 30 seconds, would be op lol.
If you have really high crit rate, your passive won't be as effective to you, e.g. you have 90% crit rate so the passive skill won't increase the probability much. But that doesn't mean really high crit rate is bad.
It's not urgent to have higher crit rate. The purpose of the passive is supposed to help you get more crits especially if you have low crit rate. I say it's as important to get crit rate on Shadower than it is to get crit rate on Night Lord.

That's how the skill is supposed to be; it'll always reset once you reach 100% crit from your base+skills.

Also, yes. Having a higher base critical will minimize the time it takes for you to hit 100% critical with Critical Growth and build your damage, keeping it high. Someone already mentioned that you'll get a butt-ton from Coin Flip. Instead of going from 60%~100% crit and having your damage vary a bit more, having 80%~100% gives consistently more high damage. Aim for a higher base.

It's supposed to do that, what would be the point of the skill if it permanently stayed at 100%? Might as well make it some OP passive skill if it did work that way.

High base crit -> higher chance of hitting a critical = more damage on monster

Shadower has a few skills that increase minimum and max crit damage, so it would be nice to take advantage of those skills by increasing our crit rate.

It shouldn't revert back to your base until you attack (meaning it will stay at 100% until you attack a monster). When you hit level 150, Flip The Coin gives you a free 50% crit rate on top of your base. You'll still want a high base crit rate because criticals increase the damage you'll deal to monsters.

Yeahp, pretty much how it works.

It's probably not ALL that necessary, but it helps to get back up to 100% and then you'll go back to your (somewhat) high base crit.