From my experience this is the defaults:

v1.x.x installers: C:\Program Files\GoG.com\
v2.x.x installers: C:\GoG Games\

Thankfully all installers do have a text box you can just quickly force the directory you want. I kinda wish the installers all had an external file they could reference in case of changes like a mini patch, or user preferences or something like that.

Yes. You can install them without compromising the security of your system.
Turning UAC off is a very bad idea, since it's similar to a Linux user running everything as root. If you don't want to move them from program files, it's much better to allow full control for everyone to "Program Files (x86)\GOG.com" so you don't encounter any write issues.

Even if I'm the sole user?

UAC used to drive me mad so I turned it off for some peace and quiet.

Yes. I can provide you a wall of text on why UAC is a good thing, but the short version is that there are certain files and processes a normal user shouldn't have to mess with, so the confirmation is a very good thing to have. Vista was overly aggressive with asking confirmation, but 7 did fix that quite a bit.

Yes I experienced that aggressiveness with Vista too and when I got 7 I switched it off almost straight away. It's probably too late now but I'll try leaving it on and see how things go when I build a new rig in the near future.

UAC is annoying, however only when it's on high. I lowered it to level 1 or 2 and i haven't had an issue, and occasionally get a popup of 'XXX program wants to make changes to your setup', and occasionally i have to force admin status but rarely.

Without UAC or non-admin control is how Viruses and malware just breed on Windows systems. Without that permission they can only affect files you have permission to change/add/modify (your user/home directory).

On 7, there are 4 settings. Setting 1 is "Never notify me", settings 2 and 3 are "Notify me when programs try to make changes to my computer. Do not notify me when I make changes to Windows settings", with setting 3 also dimming the display, and setting 4 is "Always notify me" which includes both programs and you making changes. The default setting is 3, so the only real lowering of it is to disable it completely.

I do not recall if you are on 7 or your UAC experience was with Vista, but I would still advise against lowering UAC settings.