I don't feel you're being at all unreasonable OP. It's really not fair that he's commandeered a communal space for his office. Especially if you have to go through it to get to the kitchen, is that right?
When you mention it to him and he says 'but you don't disturb me' and looks blank - have you replied 'but you disturb me, that's what I'm telling you'?
I know you said he doesn't want to move spaces, but I really think that you need to sit him down and tell him it's just not working for you not being able to use the communal space. And ask him to work out, with you, a solution.
Boosters for the back bedroom, or switching the lounge and office space, or any other idea. But remaining as is isn't an option, because it's your home too and it doesn't work for you the way it is now.
As for friends over, in the immediate short term, how would he react if you said 'DP, I'm off work for a few weeks now, and I'll be wanting to meet my mates here. They'll be coming for lunch, so ideally I want to use the dining room to eat in, how can we work that? Can you go into the office a couple of times a week?'
I think, from what you've said, the key here is he's either not getting or pretending not to get, that although he is fine as is, you aren't. And that's not OK, because it is your home too.