I don't think a hyper would overshadow the park. It would take away from stealth as the two trick pony relies on height and speed. I just don't think Merlin want that planning application battle for a hyper when the public just might not understand why a hyper is good. Bare minimum it would have to be the UK's tallest which makes it a tall ask for the planners to approve.
It is exceptionally rare in any industry for a company to only contact one supplier. When dealing with multimillion pound construction projects the law actually requires them to get multiple bids from suppliers and contractors. (They don't have to go with the cheapest, just the bid they feel works best). So Merlin working with intamin again isnt out of the question by any respect.
The issue with a multilaunch is its supporting infrastructure, they drain a lot of electricity so end up costing a fair amount to build and run.
The older rides are now pretty old and the park is going to need some serious investment in the coming years. The unholy trinity have lost their pull towards guests due to their age and should be considered for refurbishment/replacing in the next 10 years (Colossus I'm mainly talking about you). I also don't think any of the rides are pulling in people anymore. The park does as a whole due to its line up but no one ride is the headliner. Their next investment needs to be a headliner that can carry the park for a few years while they either repair or replace their ageing assets.