Good morning Whatnext.
I'm de-lurking to give you my perspective, on what is happening with your DH.
But firstly i just wanted to say, how well you are doing. You are about 100x more strong than you think you are. You kept going into work for weeks, you have been for an sti check, you are standing up to your dh's daft demands, you went for the weekend to your parents, knowing they wouldn't be that supportive. You are doing fab i think.
You haven't curled up in a ball and not got out of your bed for the 5 weeks, you have soldiered on. I don't know where you get the idea from that you are weak.
Ok, for what it's worth, my take on the situation.
He's had his head turned by someone attractive, he fancies himself in love. He has chosen to harden himself against you, because he either needs to, to stop the guilt, or to stop the feelings that he is a despicable human being for hurting someone so bad.
So he makes out that you are causing him pain (ironic) or that you and your bad things are ruining his life. This justifies how he is treating you, and why he has moved onto OW.
However, he can't just erase 11 years like that (however hard he may try) it has only been 5 weeks, it takes years to override history, so places you have been together, adverts, songs, movies, food, places etc, that he comes across will remind him of you and your time together.
Hence the texts. He will (despite his best efforts) find it hard to just go non communicado, hell he might already be realising that the OW isn't all sunshine and flowers. He was trying to touch base by sending that supposedly error text.
I think he probably veers greatly between missing you one day, to being all hard and callous (with regards to his demands) to stop the emotions of guilt and despicableness again. The fact that he sends emails and then demands you reply via a text, tells me that he either wants to keep in touch or he wants to control his inner turmoil, by being cold, hard and practical.
The thing is, unless you want this to be the continued state of affairs, you need to law down the law of whatnext. You don't need to be deliberately obtuse about it (not that i think you would, as you sound lovely) but if it's not convenient, then the answer is no. If you generally don't want to do that, then the answer is no.
If you tell him that you want the keys off his parent's back by friday, or you'll change the locks- then you NEED to change the locks.
His instant reply was to say that he will give you them in 2 weeks?
HIS HE DEAF? No? Then he is deliberately ignoring what you say. Follow through, or he will continue to ignore any/all of what you say.
IMO, the best thing you can do, is change the locks this fri, bag up his things, tell him they are going to be outside between such a time at the weekend (your son can put them out there) and tell him you are having a break from the shit he has put you through (ironic again that you seem to have bad things happen to you, and yet he has caused this one) and you will not be available to respond to any texts or emails for 2 weeks. And then just ignore, ignore, ignore. You will have a much easier ride, if you tell HIM how it is and then do it. The stupid plonk, will catch on fast.
I know stuff about the house will need to be sorted eventually, but i think it wise to switch off from him for 2 weeks ans concentrate on whatnext.
Which county are you in? You said there wasn't much activity in the local pages. I'm pretty sure you will have some regulars on here willing to meet with a tissue and a shoulder.