The bus itself will take you around 20 mins, but that's excluding waiting time for the bus. I would estimate a 30 min transfer to be safe. The last time I did this transfer around June, I just missed the bus - it took me around 30 minutes from T1 to T4.
2 hours should be okay, but IMO the T4 layout is annoying because the lounge is tucked away on one end, so if your JQ gate is the furthest one away it may be a bit tight to fit in a lounge visit.
I'm not a 100% sure of the answer this question. Logically it should be okay, because I think the Blossom Lounge is considered a "Qantas Lounge" for QF status, but who knows
When are you flying? I will be in SIN on the 13th so I could ask for you.