Can Soft-ICE be installed BEFORE Windows performs it's first boot(From MS-DOS 7.x or manually)?
When Windows wants to do its first reboot, just shut down when it's in the BIOS startup phase.
Then insert device=c:\somepath\sice-xx.exe [parms] at the beginning of the firstboot's config.sys.
(You might want to make a backup of the firstboot stage, for repeated debugging without having to install anew)
So Soft-ICE is active before the actual Windows part starts. I'd recommend to use msdos.sys BootGUI=0 anyway, makes things easier. I think the default key to get to S-ICE console is alt-sysrq. Probably it is easiest to set some conditional breakpoint that triggers when Windows enters the "interesting" stage of init.
If it is possible to have DOSBOX include a second monochrome card/monitor, use it that way. S-ICE console on mono, Windows graphics on vga. Way more comfortable!
If you have any question, just ask. I used S-ICE a lot back then, and I am in the process of equipping my retro 486 with a HGC+VGA adapter, because I want to get my DOS screen saver I wrote back in 1990+ fully compatible with W9x. (It cooperates only with 3.x, so I need to "update" it 😀 )
So I'll have to use S-ICE again and need to get familiar with it again myself...
And, be aware that S-ICE exists in various versions. It is important to use the correct one, because it hooks into some internal Windows stuff, which addresses vary between Windows versions.