Is it ok to prop up 12 week old baby to sit?

Not at all. Very young to have them be seated even on inflatable chair. Recommended from 5 months onwards when they have better support than you can.

Safe or not I duno, but my baby would slouch so we had to fill up the space with pillows to better support her. For a fun 10mins I guess it’s ok

no no no, will hurt baby's back

Not safe. Bad for the spine