Back a couple of months ago yes, Jewel check in was limited to destinations with no paperwork checks but now it’s open to all 3K/JQ locations.
As per earlier post JQ (and 3K) flights all fall under the 12 hour rule.
Technically they’re correct, once airside the only way to remain airside and get a JQ/3K boarding pass is at the T4 transit desk. Obviously the T1 jewel remains an option if eligible to enter Singapore.
I’ll try and do some searching but there was someone on AFF which I thought was able to gain access to the QF lunges by just showing a 3K booking rather than the boarding pass but this sounds like the exception.
The Blossom lounge is even listed on the JQ website for business max passengers so obviously they do. The big mystery remains, does the blossom lounge accept QF Gold/Plat flying JQ/3K in Economy, maybe if you’re still there you could ask