1. What is the advantage of installing EECH+mods over EEAH compared to installing EECH+mods+EEAH mappack 1.3?
From the
FAQ:
What do I need EEAH for?
With EEAH install you get working gauages for the Havoc cockpit. This is the only difference.
2. My 3D ingame environment/world doesn't look as fancy as on some of the screenshots. It does look better than the default EECH game but I get the feeling I'm still missing some graphic beauty. Is this because I've only flown the default EECH maps so far or did I forget about some settings?
Not sure what you're getting or expecting. The default EECH maps should look much better than they used to, and about as good as it gets.
Have you set the texture_colour etc. settings in eech.ini?
3. Does the 'eech-allMods-1.12.EXE' contain every community add-on/mod currently available or are there some game enhancements left outside this AIO pack? If so, what am I missing?
It contains all the mods a knew about and had access to when I created it. So if there are more then I don't know where to get them

But there are a couple released since the mod-pack was created, like the new Grand Canyon map (search this forum).
4. The 'eech-allMods-1.12.EXE' makes you think you're installing the 1.12 EXE but in the list with available components the installer offers, it says EECH 1.11 mod? I is cunfoozed!
Probably just forgot to update the instructions then, it does include the 1.12.0 cohokum.exe. However cohokum.exe 1.12.2 has been released since then, so you would want to get that.
Sorry to bother you people with rather noob questions but sometimes I find EECH central a pain in the ass to navigate. Imo it lacks a decent sitemap where I could veryify I've read every single piece of intel.
There is a sitemap at
http://www.eechcentral.com/wiki/index.php?title=ToC, though not sure if it lists every page yet (make sure to visit the more links too).