spells don't have a "hidden" accuracy - they are computed very similarly to weapon accuracy, you just don't get weapon-specific accuracy bonuses. if you hover over spells or abilities in your menus, or you right click on an ability and look at the description, you'll get a reasonably accurate spell/ability accuracy number. you don't have a general "spell/ability" accuracy like you do with weapons because it varies depending on ability tier and other things. [I say "reasonably accurate" because even the tooltips and right-click menus are a little bit wrong, though close; most accurate is simply looking at the combat log.]
things like "fine" "exceptional" or a club's +5 accuracy modal, or even most magical weapons +accuracies generally only affect weapons. weapon penalties like the pistol proficiency modal or blunderbuss acc penalty only affect weapons as well.
however, shield accuracy penalties (medium or large shield) affect both weapon and ability/spell accuracy. heavy-duty offensive casters should not hold heavy shields.
yes, for the non-probability savvy, what this intuitively means is "what is the chance that any of your hit to crit triggers" which is equal to (1 - "what is the chance that none of your hit to crit trigger"), hence all the 1-'s to "invert" your hit->crit odds from success to failure, and then one more flip to go from .81 to 19%.
assassin only gets its bonuses from stealth, which--once you enter combat--is impossible to become re-stealthed like you normally can (unlike games like BG2/IWD where if you ran out of line of sight you could re-stealth). You have to rely on abilities like Smoke Veil or potions of invisibility to become re-stealthed, which is expensive and pretty limited. i and others tend to recommend multiclassing assassin with a caster so you can really get mileage out of those assassinate bonuses by using them on heavy-hitting spells instead of a few weapon attacks.
by contrast, streetfighter bonus is actually fairly easy to get really high uptime on... and the numbers involved are huge. -50% recovery time bonus is roughly equivalent to getting +33 dex through your recovery... on top of that you also get huge +50% sneak attack damage bonus. if you manage to get both bloodied and flanked, you also get massive crit bonuses.
the problem is that streetfighter bonuses--while they can be easy to get--also encourage extremely dangerous behavior. being flanked and bloodied is generally a bad situation to be in for a class as squishy as a rogue.