Thawing vacuum-packed fish
Typically the plastic used for vacuum packed food is polyethylene, which has a low melting point, depending on the plastic it could be as low as 110°C, or 230°F. Once it melts you have liquid plastic on your food with the nasty flavors and potential health concerns.
The big thing here is microwaving can very quickly cause the plastic to reach that temperature (actually it raises the food to that temperature which heats the plastic), but explaining the dos and don'ts is complex so they just make a blanket statement so they can't be blamed if something goes wrong. In reality you can safely thaw fish in the plastic using any method you want as long as you are careful not to reach that temperature. If you microwave thaw do it on low and check it often to make sure you aren't overheating it.
Regarding re-freezing I won't cover it here as there's already a very good question and answer on this site here. The short answer is refreezing can be done safely as long as it's within certain parameters.