I think an important thing to remember is that, just because children don't learn something (like not hitting/picking nose/whatever) right now, doesn't mean they won't learn it at all. Expect it to take time. The point is, though, that punishment doesn't work either, so no point in thinking 'I can't 'do' TCS - they're still hitting' when punishment just teaches them all the wrong things and doesn't work anyway!
I think food is a big issue - don't make him porridge unless he asks for it? Let him self-regulate his own food? Mine can have what they want, when they want, and their diet is really, exceptionally well balanced. It's really, really, really difficult to let go and let them do that though, to trust that it will work. In what way were you giving him choice before? Like saying 'would you like this, or this?'? I leave it up to them completely, either to ask me for stuff, or for me to say 'right, what would you like for lunch?', for example.
However, we do eat together as a family so I make one main meal a day. I try to make things all the children like - and I do meal planning, so I write up on the calendar what we're having so they know what to expect. I don't make special, extra meals, but if they don't want what's on offer, they usually have cereal instead, which they're happy with. And then, quite often, they have a plate of whatever I cooked anyway!
I wouldn't bother with saying 'calm down' - have you ever found that it helpful someone saying it to you when you're wound up about something?
So....personally, I think I would have done what you did in reality, but what I think I would like to have done was:
- not say 'stop doing that' but say 'here, could you throw these instead please because those hard things might break and/or hurt someone'. Starting out with a 'don't do that' thing can put a child's back up straight away, so putting a postive 'do this' spin on it may have stopped it all getting out of hand
- Have plenty of snacks on hand for him to eat if he gets hungry later and cereal doesn't fill him up
- We only bother making food that our DC's have asked for (apart from the evening meal, when we only dish up what they've asked for) so that we waste very little, so I wouldn't have bothered making the porridge (btw, can porridge be refridgerated and re-heated later on?)
Room tidying: I think there's stuff on Joyfully Rejoicing (link on the OP) about chores and children. I don't bother with incentives, but my children often respond to a timer - let's see how much we can get tidied in 5 minutes? Or I accept that I need to do all the putting away, but the children can at least gather it all together for me, so 'how much stuff can we get in this ikea bag?' (that works particularly well because then I can chuck the ikea bag in the spare room if I don't get all the stuff put away in time