An easy way of telling if the TVIS is not opening up is to simply unplug it. This should leave it open whatever the revs are at. It should feel shittier down low (assuming it was working before) and then rev like normal.
If the TVIS isn't working, it's normally stuck open though. I haven't read anything about it being stuck closed.
As for spark plugs, I just use those Bosch items listed in their book for a 4AGE motor, hasn't cause me any problems so far.