Your first paragraph said “ you had accepted the situation” as you felt it was either that or not have the relationship go any further...
That should have been the red flag, and the time to question the relationship before you got so involved, but love can be blind no matter how old you are.... but you waved a white flag..... so unfortunately this gave him “control” of the situation.
Moving forward now that 10 years have passed and you have invested a lot of your time and effort into the relationship and maintaining the house and improving it, I would think that by this time any thoughts on how “equal” and valued you feel in this relationship should have been on your OHs mind. There is only one person able to plan in this relationship, and it’s not you. I’m not sure if your “failure to engage in the renovations” is more down to you now looking ahead at your life with him and wondering , is the extension a project or way to make you “feel more included” while at the same time improving his (and his families asset) I’m not sure if his reluctance to include you either by marriage or till you die if he goes before you, is really down to his (bossy kids, who really don’t have a say) or if that gives him an excuse not to share or include you in a way that would give you more security, without it looking like he is the one holding back.
I do totally understand that this is his only home and he does want to do the right thing by his family, but does he not view you as his family after 10 years together?
I don’t agree his family (however bossy they seem) should not benefit, I also feel that the only person currently benefitting in this is your daughter... as it appears to sound like it’s “rent free”. To resolve this you have to firstly have some discussions with your daughter in relation to you returning back to your house without much if any notice ( should the worse happen to your OH) as the last thing you will need then is conflict within your own family. You seem to have accepted his family just want instant access to his entire estate, so you can’t keep going along with this while miscalling them for it, as he is the one who is giving them these assurances, that it’s his wish for this to happen, so obviously unless he lets them know his feelings in the matter have changed and you are important to him and should be given some security, whether they like it or not, then you can’t blame them for trying to protect what “he had worked hard to build”. I wouldn’t go behind their back, I would have a discussion with him about how can “he help you” resolve this, and if he agreed in a way to include you, that you both discuss it upfront with them, so they don’t make bitter accusations towards you if he went first, get it out in the open. Maybe if he sees it gears their tone, it will be the push he needs to do something to protect you.
You on the other hand can’t have half the cake, and another cake in the oven with your own property, so this needs to bring some income into the home you both share, to make everyone feel it’s not “ all take and no give”.
Sounds like you are not as secure in the relationship as each other. Good luck, I don’t mean to be harsh, but with second marriages sometimes someone pays for the exes mistakes.