*First, if I compile intelfb and fbcon into the kernel, and no other 
frame buffer modules, I don't think the "vga" parameter is ever 
processed. I get a blank console and the famous message   intelfb: Video 
mode must be programmed at boot time. in the log.

That is correct vga= is only used by vga console or vesafb. What vga= does 
is pass a mode that the BIOS can set.

(Side question: Should CONFIG_VIDEO_SELECT really depend on
CONFIG_VGA_CONSOLE?)

No. CONFIG_VIDEO_SELECT is used with vesafb as well.

*If I additionally compile in the vesafb driver, then it seems to grab 
the console despite the "video=intelfb:..." command line argument, and I 
get a working vesafb console.

That is correct behavior. If the native driver fails then vesafb will 
start. I bet the mode you can passing is incomplete. Please try 
something like this

video=intelfb:mode:1024x768-8@75