Argh this just sounds like a ticking timebomb.
I cant say anything that PPs have already said. About everything, especially the boundaries issue. Its a bloody nightmare.
To put it into context, my mother rented a house from someone who had lived there previously. It had been their family home before they built a new house.
They gave carte blanche to my mother to use the property as she pleased (obviously within the T&Cs of the letting agreement) but kept popping over and commenting on how things had been re-arranged 'we didnt like it like this' etc.
When there were problems with the property they got very arsey about them. They seemed to take it as a personal slur rather than as a legitimate complaint. 'well if youd just done this like we used to then there wouldnt be a problem'. They still saw it as their home and didnt like other people doing things differently and when there was a complaint they took it personally.
When someone is invested in a property (emotionally or financially) there will be problems.
Imagine if you decided that instead of a spare room, youd like to turn it into a sex dungeon and have a local band over each weekend (within noise limits of course). Do you think shed veto it by saying 'well I did pay for this, I think I have a say'? Its a tongue in cheek example but unless this is strictly business with a clearly worded pre-arranged agreement drawn up by a solicitor I would not touch it with a bargepole.
Go to a solicitor, check out your options but make sure that DP is going to back you up should there come to a disagreement between you and MIL.