Ugh -- it is obvious, isn't it? I guess you can chalk that up to a brain cramp. 
I guess I'll have to decide if it is worth spending $15 on the 4.15 ROM, as it seems pretty iffy to me whether or not it will fix the problem since the 4.05 ROM is dated 1992 and 2.xx was released in 1990.
The only thing I can think of is that the ROM is defective. A 4.(0)5 should work fine with Kickstart 2+. If you need a 4.15 (never officially released BTW, the last official ROM from GVP was 4.13) PM me and we'll work something out.
What happens if you take the Multistart out of the equation? I mean, take the 2.04 ROM from the multistart and put it straight in the kickstart socket?
If you want to know if it's the 4.5 ROM that is causing problems, remove J9, it should disable SCSI.
Set a jumper J3 to disable the MMU if you want to know if it's not some kickstart remapping problem (unlikely)
If you have a rev 4 board (GVP) check CN15 and make sure it's set at pins 1-2, not 2-3.
