You can put the adapter back in after you remove the blown tant. I know it's more risky running without the filtering but it should still be fine. The blast mark you saw on the main board was just residue from the tant when it was still smouldering. I had the same marks on a 5160 when it started blowing up tants all over the ISA bus but it still worked fine.

I owned a IIsi once and I do recall that the onboard video was very picky about monitor frequencies and resolutions so I just stuck with a mac display and got around all this multisync monitor BS.