Perhaps the reason no one read the readme was because it was buried in the pk3 file. Not everyone has the ability or knowledge to open pk3s (I use PakScape myself because it works so well) so putting the readme in there is not a good place for the general public if you want it to be read. 90% of the maps I download either don't have a readme, or the .zip file contains a pk3 and
a separate readme file. Just something to consider for the future.
Now to address the issues that you addressed.
The best way to figure scale in Radiant is by using one of the spawn points as a guide. They are roughly the height of the player in game, so when building a map it's best to keep one of them in your field of view until you get a feel for the scale.
As I said earlier, the handrail/banister things for the stairs should only come up to about half the height of the player (about 32 units).
As to the Mergovingian, i'm not going to argue about foreshortening, but if you want to keep the doors the size they are the texture will have to be reworked, since that metal plate is about head height (and in-game is much higher).
If you want ambient light throughout your map, it's actually 2 commands... add the key ambient
and the value (the value is similar to the value for regular light entities... don't put ambient light up too bright, though. Anything over 50 starts to look ridiculous). Then add the key _color
and the value of a 3 digit RGB color (1 1 1 would be white light). Easiest way i've found to normalize RGB color values is to hit 'K' in Radiant to bring up the color wheel, pick my color, then copy the values for R G B.
I'm not sure about the yellow. I like the color scheme the way it is. The only thing I want to mention (again) are windows. There should be windows on both sides of the hall (close to the large doors).
Keep it up, it's improving.