It is a misconception that they were forced to convert. They were warned of consequences. There are two possible outcomes after اتمام الحجت. Either the قوم heeds the message and they are then given the dominion on earth. The alternative is they reject the message. In that case they are purged from earth (through angels or other اقوام). Meccans accepted the message except for around 58 of them, who were purged.
There have been around 300 رسل (it is so believed; though around 25 or so may have been named in the Quran; more may have been mentioned in earlier books). The big picture is that God created a world with physical laws and is testing us (and jinn) imbued with moral code to see if we do good or evil. That judgement will be made at the end of of time on the big judgement day. But to help us understand the concept, God sent around 300 رسل to hold mini-judgement days. The last time it was done was in December 629 CE. In this case, they were given dominion on earth, since they accepted the message. Had they not accepted, there would have been no point in waiting till the final judgement day, so had they chosen the alternative, they would have been purged.