If combat has started and the other players start discussing what to do to me can I play a one-shot card while they are discussing?
Yes, go right ahead. For most one-shots, the only requirement for playing them is that there's combat happening. As soon as it is, you can play your one-shots. You don't need to wait your turn or anything like that - there's no turn to wait for! Snoozers losers, you've got loot and levels to find.
From the original rules:
Items – Using Them
Any one-shot Item can by played during any combat, whether you have it in your hand or on the table. (Some one-shot Items, such as the Wishing Ring, may also be used outside of combat.)
There are no other constraints, and no "you can only play it when the other players say it's OK or are done talking."
As for Curses in particular, remember:
Curse cards may be played on any player at any time. ANY time, do you hear me?
Also, don't forget you don't have to wait all that long for them to decide what to do, version 1.6 clearly states:
When you kill a monster, you must wait a reasonable time, defined as about 2.6 seconds, for anyone else to speak up. After that, you really have killed the monster [...] though they can still whine and argue.
So go ahead and kill it while they discuss, you'll either force them into making a mistake, or get the treasure ;)