Perhaps you can’t really skip the level previews in SMB1/2J, but they can probably be hidden
now, curiously, I did run a seed with SMB1 8-4, and Mario’s movements played out fine for me…
Viewing post in Super Mario All-Stars Crossgame Randomizer comments
I was wrong to say that the SMW physics always prevent you from proceeding, but it seems to occur more often than it doesn't for me. Unknown factors seem to affect where Mario will stop on the platform. You'll first notice something is wrong if Mario stops in the air one tile to the right of where the higher ledge of the platform ends. He will run and fall off the lower ledge before he can jump. Sometimes the score tally will start before the death is detected and the cutscene will proceed as normal with Mario under the lava. In other cases, I've seen Mario stop on the very edge of the upper ledge or gravity take effect before he stops. The cutscene will proceeded as normal in both of these.
Seed: fa963; SMB1 World 8-4 replaces World 6-Fortress 1. I had to make a savestate and resume on a version of the hack without the SMW physics.
I did recreate the 8-4 death bug while testing on real hardware and will resolve it soon.
I actually originally did skip the SMB1/LL level previews while building the first version, but it caused some weird level loading issues that I didn't feel like chasing down so I just restored it. I actually kind of like the level previews and the only reason they're not in SMB2U is because it's part of the character select screen, but I guess if it annoys people I could work to remove it or hide it.