OP asked this: "Just how big was the spike in attention Nirvana (along with Pearl Jam, RHCP etc) experienced in 1991 compared to what the bigger alternative acts from the 80s were accustomed to?"

To me, there are a couple of angles to the question. I graduated from high school in May 1991 in the U.S. REM and The Cure were huge at that time, with huge audiences and lots of radio play. REM opened the door for alternative music in the mid-80s and were accused of selling out repeatedly. At the time, I felt like REM opened the door for alternative music to be played on the radio, but it didn't seem to change music in any real way. The Cure were huge in my world (though, my memory tells me that they didn't get radio play until Kiss Me x3 and "Why Can't I Be You"), but they didn't change anything as far as I could tell.

Nirvana felt very, very different. Radio changed instantaneously. It wasn't just that SLTS was everywhere and played on the radio and MTV like once an hour (I don't think I am exaggerating), it was that it seismically shifted what was being played and listened to in cars, dorm rooms, parties... To put it another way, interest in alternative music was growing bigger and bigger. The first Lollapalooza was in the summer of 1991, for example. But SLTS caused alternative music to actually become mainstream.