Never tried tubeless without the nut, but my assumption would be the nut is very, very important. I would absolutely put a tube in until you can get a proper tubeless stem of appropriate length.

IMHO, your LBS is mucho loco. If it doesn't work loose by itself when the tire is low on air, the stem will almost certainly get pushed into the tire when you (or someone else) tries to add air.

You aren't crazy, this is a really bad idea. Not only do you definitely need the nut, using the valve extender pretty much guarantees that you're going to have a sealant blockage that prevents you from inflating it at some point down the line, probably at the worst possible time (of course).

---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

I would never return to that shop, that set-up is "nuts" to use a term.

BIG caveat, is if the shop somehow bonded the valve in place inside the rim, but even that is a dodgy set-up.

These guys make a 105mm valve at the long end and shorter offerings from there.

---

"When the going gets weird, the weird turn pro."
Hunter S. Thompson

You should just be able to push the valve back into the tire… what would even hold it in without a nut?!?! This is why I do most everything in life on my own. I don’t need a 16 year old wannabe mechanic kid killing me.

There's no way the valve would seal without the lock nut unless the valve extender is thicker than the valve hole. Perhaps they figured out a way to make it work. I don't like extenders as the valve sometimes freezes using CO2 and won't seal. I would order a 100mm valve from Amazon or have the bike shop do it and re-do the setup. You could even get 110mm if you want a bit more length.

Not sure if 90 will be enough, but just bought some RESERVE FILLMORE valves suggested by another poster. Expensive but no valve to clog with sealant. I did find I could not use the Park Sealant injector with it because the hole was too small, so had to dump the 2 oz of Stan's into the tire, rotate it and pop the last bit of tire back on. Nice thing about RESERVE FILLMORE is you don't have a valve core to bend and start leaking & it won't bend like a normal presta valve.