Such a huge magnetic force would have wiped Miko's phone clean, yet it functions perfectly after the magnet has been shut off.
Miko's phone shouldn't have become magnetized, since it's mostly made out of plastic.
Arcee and Bulkhead decide not to finish off the unconscious Decepticons because they don't want to risk another incident with their magnet weapon. However they still could have used their regular weapons.
After the robots realize they've been polarized and thus can't touch metal, Bulkhead instantly manages to grab Airachnid, who is made out of metal, and throw her at Breakdown.
If Arcee truly has become so magnetized to Bulkhead that she cannot get off him, then her limbs should have been stuck to him as well. Instead, she can move her arms and legs around perfectly.