Subfloor Replacement Options
MDF, particle board??????????? What happended in there?? Neither of those is a good underlayment, as you already know. I don't see how you can patch part of it, sounds like that has already been done. Tear out down to the subfloor plywood, seal it with shellac or a water based sealer, then lay another layer of OSB or plywood on that, glued/screwed to the subfloor. That will strengthen things up an provide a good suface for what you want to put on next, carpet, laminate, wood, etc.