The note about vres is just so that you don't draw only the first line, right?
But it says that all three char modes differ only in the required memory for the matrices... Are there no other features?
Yes, because in this mode the character map is read from the memory for each scan line it is the same, and the pointer is changing in the character set.
Yes,
in CH256 mode you can use color0,color1 pair from the palette
in CH128 mode you can use color0,color1, and color2,color3 pair from the palette
in CH64 mode you can use color0,color1, color2,color3, color4,color5, and color6,color7 pair from the palette
in 2 colour mode.
There is a feature also, in 2 colour graphics mode you can use more colours by using MSBALT, LSBALT bit of margin bytes, and if I remember well ALTIND0/1 has also effect, but may be i have wrong.