Hello Technium,
Let me try and help you . . .
Your problem seems to be easy.
For sure, there's some monitor driver being activated in the startup, that's out of the range of your monitor/TV, and that's not being processed by your Scandoubler as well. There are some weird resolutions on the Amiga world :-)
Well, just boot with no startup-sequence.
Once in the AmigaDOS screen, typethe following lines, followed by an enter:
cd devs:monitors
makedir temp
copy #?.#? temp
delete #?.#?
dir
Just confirm if the "temp" folder is the only file showed by the dir command.
If there are more files, please copy them one by one to the same temp folder, and after that, delete them.
Now reboot your Amiga, preferably running on your TV.
The Workbench screen "should" appears.
If it's true, follow the instructions given by Boot_WB, for installing the Picasso system again.
By the way, let us now what's in that devs:monitors/temp folder :-)
Regards.