1. No. Digi bb only stand alone use, cannot combine with your postpaid sim or prepaid sim.
2. YES. Look like your place don't have 3G coverage.
3. Normally sign up at road show will give you RM88 for explore package. But you don't know where it will be there. Discover normally no promo, standard RM58 even at road show.

answering to yr qyestions:-

check your coverage at www.digi.com.my/broadband

1) wat simcard do u mean? the current phone simcard? broadband simcard and normal call simcard is different
2) yes. if the signal is bad, it will drop to edge
3) you can try visit dealer in lowyat for the promo of RM88.

look like you want broadband to be use in your phone to get online.
RM66 for EDGE data unlimited to your postpaid. For me not worth it if only get EDGE speed.