i got h1b visa 221g at mumbai consulate in india on 30th sept and have submitted the req documents on october 12 and am now currently working in singapore(i left the US on may 3 2010 and i have student visa stamped till 2014)

i want to know my options of coming back to US
and visa stamping

if i can withdraw my h1b 221g at mumbai and apply for h1b at singapore?
if i change employer and apply for another h1b can i go for stamping at US consulate in singapore
if i can come back to US on the same student visa stamped with new I20(i been out of US for more than 6 months)