The high side drive circuit probably cant' work that way. When Q5 or Q7 are switched on, part of the load current is flowing through the transistors for some time.
Correct. MOSFET driving voltage is referenced between gate to source.
IRF840 is an 500V 8A continuous drain current, >8V Vgs
I don't now about their power capability, the type is unknown to me, but an effective current limiting mechanism should be present to my opinion. Switch-on operation is rather slow due to 4K7 resistors (I guess 5 - 10 us), but this may be O.K. for 50 Hz operation. Paralleling MOSFET without individual gate resistors generally raises a risk a RF oscillations during switch on-ramp.