This thread is getting too long and unweildly, but... did you try that? I don't know if the IIsi is one of them, but a number of 68k Macs won't produce video unless they have a working PRAM battery. (LCs fall into that category.)
Googling around it does appear that the IIsi uses composite sync rather than vga-standard seperate horizontal and vertical sync. Some (many?) Multisync CRTs can accept composite sync on their hsync pin, but since I don't own any machines that old I can't comment on whether that might be a feature that's becoming extinct. If you were looking for a monitor just for this purpose that's not the original equipment I'd recommend a medium-to-high-end early-mid 90's vintage NEC or similar. Anything made after 2000 or so I wouldn't *bet* on unless it was really high end. (IE, was made to work with UNIX workstations or the like.) A composite-sync friendly monitor is all that will work with a "gutless" adapter like what you have. They used to sell adapters which contained active components to seperate the sync signals so you could use monitors which didn't support composite sync... you may have a challenge finding one. (Apple apparently sold an adapter *themselves* to allow the IIsi to work with newer Apple monitors. Stacking one of those with your switching adapter might work...)
Or your monitors and adapter may be adequate and the machine is simpy dead. RIP?