R.e the holes in the wall. Make up some lime mortar with some 3.5nhl (hydraulic lime) Mix 3 parts sharp sand to one part lime and mix to a fairly stiff consistency. Fill the back of the hole and the bottom with some mortar and start putting some rocks in. Fillings any gaps with more mortar. Use your hand but make sure you wear latex/nitrile gloves. That'll be fine. Don't use cement. I agree the chipboard will help the bounce as you are securing each joist off each other with sheet material. It doesn't seem to me as a proper "repair" though. In an ideal world you would repair the joists, fit noggings and sister where needed. Then fit floorboards down. If this would done properly there will be no bounce and you have fixed any issues for the future. At a push why don't you repair the joists as above and then fit the chipboard. I don't see any issues with the weight on the joists. But maybe upload your joist size and someone will dig out a loading table. Have a cup of tea and take a breath. It'll all work out fine Edit: just seen you have planned to glue the boards onto the joists. I wouldn't think this a good idea at all for any future access. Anyone else?