Just wanted to say I really feel for you. It is a tough journey, and worse if your MIL is not understanding.
Just a few thoughts, not meant at all to minimise what you are going through, but to see if there are ways of easing it a bit ... how allergic is your DS? obviously anaphylactic, but does he react to airborne particles or touching a tiny bit of something? I guess I'm thinking your panic sounds rational if that is the case, but if not then yes you need to be careful but as long as you are monitoring him and his food intake/contact with food then you can probably learn to relax into that a bit.
Agree with giving your DH more responsibility. Does he come to the medical appointments with you? could you get the consultant to outline for him the precise risks (ie death), dangers of cross-contamination, and the issue with other nuts etc. He might believe it more if he hears it direct from a professional, especially if they are a bit stern and horrified at his actions.
No way would I leave your DS with MIL. If she won't notice a problem then it's a definite no-no, even aside from the fact she might not be careful enough. Though a bit of education for her might not go amiss either - maybe she could come to an appointment too.
It is scary taking all that responsibility, but in the end it's the cross you have to bear. We have occasionally made a mistake and given DS milk and it's awful hearing your child say 'why did you do this to me, Mummy?', but in the end you are only human and you can only do your best. If the worst happens, well you have the meds, you call the ambulance and he will almost certainly get through it. If it's any reassurance, every single time we have made a mistake has been when we are on holiday and the milk packet looks different, or once I put cow's milk in a glass in the fridge for DS2 and DH gave it to DS1 while I was asleep. So it was always when we were out of the normal routine - at home you do just get into a system that works.
For CPR, I think a 2yo is way more robust than a baby. Have you done a course? Again, I think you need to trust yourself to do the right thing. I'm sure you are a lot more competent at all this than you feel :)
To be honest, I think the 2yo phase is probably the worst period, as they are touching everything and don't really understand. They learn really quickly once they are more verbal though - I was terrified about sending my DS to school at age 3 (in France so earlier) but by then he would already put his hand up and say he couldn't have chocolates etc if they were being given out. It is still tough at times, but I now have a lovely mature 7yo who is incredibly sensible about food and has an ability to say no that far outstrips his peers, simply because he has had to. He is also pretty good at cooking - I always found the food preparation stuff overwhelming, but used the cakemaking as an activity for us to do together.
Your friends don't sound like they are much help. Do you have at least one who really understands? if not, how about joining some kind of allergy support group? I did that once - didn't really get on with it, but it made me feel my son's allergy was actually easier to cope with than some of the ones in the group. I actually find this board fantastic for support about specific problems or even a general moan too.
The other thing I wanted to say is that your feelings sound utterly normal and don't feel bad about them. That said, when I was in hospital with DS as a baby after an asthma attack (2 weeks after his first allergic reaction) and feeling very sorry for myself, I happened to wander past the child cancer ward and that really put things into perspective. I try to remember that when I am feeling down.
Sorry this is so long!!! but hope some of it is helpful.