I am no expert, but as I understand it...
1. If a Genestealer can only approach a Marine and runs out of APs, it must stand there. The Marine can react if allowed as normal, but is likely to blast away at the Genestealer in its next activation.
2. There are no multiple combats, as miniatures are activated one at a time. In your example, one of the two Marines would activate first - probably whoever is behind the Broodlord - and carry out all of his attacks, before the other Marines would do anything.