Hey, welcome back.
1} I don't think so - US and EU codes aren't interchangeable. At least not for booster packs, I'm assuming the same applies for games themselves.
2} Not sure {will check when I get home} Whether or not you own GR is irrelevant - the resources for the expansion are installed with the base game, owning GR simply lets you access them.
1. Not sure on this one. I believe you can only use a US key on the US client, and EU key on an EU client, though I may be mistaken.
2. My folder is 3.93GB's, but I believe a small portion of that is MA arcs and saved costumes. The launcher reads it as 3.7GB's, with GR installed.
3. I would, but I'm on the other side of pond.
1)Fairly sure of this TBH, I didn't see any reason why it'd change but thought I'd ask just in case.
New question:
4)How big was the download for i18/GR? If the total size for the whole game is only 3.7Gb or so then GR can't be that big...
Have found an old i16/i17 (not sure which) install on my laptop so that should speed things up a bit. 
The pre-download was about 670MB's, plus a little for things like today's patch (or I guess yesterday now XD ).
Long time since I was last here, been off playing other stuff and doing things more important than PLing another Fire/Kin (they been nerfed yet?). Since I last played the HDD that had my mostly up to date CoH client died (well more killed) so now I want to come play Going Rogue I have some issues.
I didn't realise that it was out so soon and consequently burnt through my entire yearly (it seems) downloads getting WoW and Oblivion running after the HDD loss, now I can't get the entire GR thing as a digital download. EU doesn't seem to have a non-digital version. So some questions:
If I buy the Complete Edition from Amazon.com I'll then have a client that should only need minor updates until i19 but will I need a different code to activate my EU client for GR?