Really cant see that the monopoly commotion would get involved
the demise of american adventure and camelot show its an industry that is in decline makes no sence for the government to butt in and say you cant buy that let it go bust and every one loss their jobs... ...
And yes they can open as many parks as they like (subject to land and planning consent)
That said DM have a solid trade and a current long term business plan so I i think it will not get sold any time soon
They would get involved in regards to Drayton Manor, like you say it's got a solid trade and is doing well. Merlin wouldn't be 'saving' anything, rather they would be launching a hostile takeover.
In the cases of Camelot, American Adventure, etc. Because those had failed, Merlin would be looked upon more favourably as you'd be keeping jobs and supporting the local economy, a takeover of Drayton Manor does neither of those and serves only to further Merlins interests.
All that being said, Merlin have zero interest in opening or acquiring any more theme parks at this time. Midway Attractions (Sealife, Dungeons, Legoland Discovery Centres, The Eye Brand, Shrek) are far more profitable with minimal investment and no major ongoing costs. This is how the company intends to expand for the foreseeable future.