Honestly, I think they're a combination of issues here
Obviously there's the himself retreating into himself and you having to take on additional parenting duties because of your husband's disabilities.
But there's also that him working from home at all seems to be a problem to you - it was before, and even more so now.
Yes, in the longer term you need a better solution so you don't feel the lounge is out of bounds, but it doesn't sound like he's demanding that. You've not mentioned him banishing you for meetings, getting huffy when the kids play, or spreading work stuff across the whole lounge. More him quietly getting on.
You could be juggling mostly solo parenting with being the only earner. It's actually amazing that despite what he's going through, he can keep working, keep earning etc. If he loses this work, he may not be able to get another job, and he's managing it as best he can.
You and the kids could chill out in other rooms (the garden room? Bedrooms) or you could have a blanket 'no working from 3-6' rule.
There are tasks he can do to help with your load, but he might not be in the right frame of mind to identify them. Having children read to him/him reading to them, doing the online shop, that sort of thing. If he gets paid overtime for the extra hours, the that could be ploughed into getting extra help in.
But I don't think it's reasonable to get grumpy about him WFH in general, and it's not surprising he's not himself in mood. It's a big adjustment for both of you, and you have different burdens in that regard.