Auto G/A is only available if the approach is carried out with both autopilots engaged. This can only be done for an ILS approach with both NAV receivers tuned to the ILS and it is only done when an autoland is planned. The reason that all ILS approaches are not routinely flown with both autopilots is that when both are engaged, 10 units of nose up trim are automatically applied at 390' and this then makes for an abnormal trim setting if the autopilot is then disengaged.
Fuel in the hold - it's in the 'Performance Inflight' part of the QRH. For a typical landing weight and holding at low level (<FL100), the fuel flow per engine is around 1200kg/hr, i.e. 40kg/min total.
Performing a diversion? Go around, re-tune the NAVaids for the missed approach and climb to a safe altitude, arrange with ATC to route as directly as possible to the diversion airfield (taking terrain into account), re-program the FMC, re-set the pressurisation controller for landing altitude and cruise level, talk to the company ops if there's time, brief the cabin crew, brief the pax, talk to the handling agents at destination and diversion airfields. Land at diversion airfield. In terms of fuel, the legal minimum to land with at diversion is Final Reserve - 30 minutes holding fuel (1200kg for a 737 classic).