All sounds familiar mairimac!... my DS can be like this, and just gets so bossy - he will continually insist on a different spoon/cup/plate for example and have an almighty strop if it's not the right one - even though I don't stand for it, it's SO tiring. Another thing he does is if he realises something is naughty he will just do it and do it and do it just to push me and see what happens - often with an infuriating smirk on his face! And those days of non-stop fussing and stropping - aaaarrrrgggghh!
I also find it comes and goes, and the worst times coincide with growth spurts. (I wonder if it might be that he's actually physically in pain at those times...)
I've concluded that a) once you've started a battle, don't give in. Eg with deliberate naughtiness, he will not get his way, whatever he does. b) don't start battles unless you have to - eg wants to wear wellies, let him wear wellies, whatever. c) i can sometimes distract him and defuse it, by giving him choices, also with humour.
My most recent idea is having a whingeing contest - I say "Oh DS good whinge! But I can do better . Now your turn!" He does a whinge, I do a better one with foot stamp, he copies, etc. It soon makes him laugh and then very quickly (because it's no longer winding me up) it loses its appeal.
I also find (though mine is slightly older) that it works well to say "Oh dear, what is that terrible whingeing sound, can you hear it DS? If I could hear a nice big boy asking me nicely I might be able to hear better!" etc. He has now grasped the concept of asking nicely (saying please and in normal voice) and that it gets better results than whinging and stropping - he just needs reminding!
However after a day of it I am bloody exhausted. I cannot imagine what it's like having a small baby as well Mung - hats off to you.