Stella:
I think with your MIL like that, abit hard in the long run. As always, I am quite extreme and what I would do if I were you is to put my foot down. If she wants to look after your son, she has to do it properly, i.e. no galavanting excessively, no disappearance, no making you take leave. If not, you will move out and cut off their access to your son. This is very extreme but if done well, it may work. Otherwise, you will always be at her mercy. YOur work will be affected lor.
Re: Feeding solids
To succeed, you need to try abit each day. Babies have moods so even if they don't want it at a certain times/days.