but still... a powerlevel post
Well, like said before, that is bound to happen when someone pops up a "who will win if
they fight?" question. You know, ifs
can trigger much more reaction than you thought possible. If
you don't like powerlevel discussion, why bother asking "what if" in the first place?
As for Gil, Independent Action is a matter of tying oneself to this World without Master. It has to be noted that Masters aren't simply Prana providers to Servants. They give them connection, ties that can make them stay in this World; that is the most essential part of Masters. However, with Independent Action, Servants (especially Archers) can exist even if their ties with this World are severed, assuming they can find adequate mana supply for themselves. Having a real body means he can no longer go back to spirit form. Of course, the anime seems to forget this.
And yes, we can safely assume that having a solid body means Shiki's eyes can kill him.
What is the difference between his Heroic Spirit status with and without Independent Action? Well, from what I can figure, being immersed in the Grail and having a real body automatically grants a Servant A+ rank Independent Action: ability to exist without Master. This is only speculation, though, as Gil's IA rank at HF 4 is A.
As for spathi's question, Ciel's already answered it.
Shirou can do hax with Servants because he uses Noble Phantasms
Seriously guys, if you can't stand powerlevel discussions, don't even bother asking "what if". Only suggestion.