From a review:

"The portion of the heatsink covering the 790i SPP is extruded aluminum with a coating applied to give the illusion of copper. The integrated "Fusion" water-cooling bowl is covered with a thick copper finish inside and out ensuring there will be no issues with galvanic corrosion due to dissimilar metals."

So, it's not really copper, it's aluminum covered in a 'copper finish'. Personally, I wouldn't trust it to not have small flaws/scratches/etc... but that's just me.

As to your questions, if you are going to mix metals:

Yes, you'll need to use a corrosion inhibitor

An inhibitor will slow down corrosion, it won't completely eliminate it.

Done correctly with the right mix concentrations, corrosion can be slowed down enough that you'll likely have replaced everything for newer components before it becomes an issue. (Done wrong though and you'll be replacing everything a LOT sooner than you planned on )

Yes the fluid "wears out" and needs to be replaced once in awhile (I'm not 100% on this, but 3-4 months seems to be about average as opposed to 6-9 months for non-mixed-metal loops).

Yes, if you're mixing metals and have to deal with possible corrosion, you'll be increasing your maintenance as you'll need to flush the loop and clean your blocks more often.

I can't/won't recommend a particular board, but from the reviews I've read on this board, lots of reviewers seem to think it's a good board but not quite up to snuff for how expensive it is...make your own decisions there.

On a personal note though, I think trusting ANY 'stock' cooling doesn't make much sense. Heck, even air-cooling, I'm going to take apart (and/or replace) any heatsinks/fans. To correctly apply the right amount of tim at a bare minimum.....