Thank you!
Uses up a turn: Yes. However I am not always so strict with turn order myself when running MoSh so being a little loose and letting players have "extra actions" would probably be fine if that keeps the pace up and makes things interesting.
More melaphages causing multiple instances of harm: no. I might go back and edit it so that you get [-] on the save if you have more than 1 attached though.
(When I ran the game, there was 1 incredibly well equipped Marine with a bunch of guns while everyone else only had nominal weaponry. I ended up ruling that everyone Close who got melaphage'd would just try and get in front of the Marine's gun barrel and wrestle him to force him to pull the trigger haha)