Are you sure about this? That someone can't merit a single post more than 50 merit. There is no limit to how much you can merit a single post, but there is a limit to how much you can merit any user each month. You can see a post that deserves even more and decide to merit the post 50 merit each month. There is no limit to how much merit a single post can receive.
That's incorrect. You can't send more than 100 Merits in total to a post.
You are right and only 50 can be sent for the duration of 30 days or 1 months after which that user can be able to still top another 50 in that same post.
I have seen some post having 100 merits from a particular user but they don't earn the 100 at a go but after one month gap.
By the way from OP, I will advise that many merits source aren't holding back their merits sometimes it's actually very hard for them to keep finding quality post to merit but they can't just dump such amount of merits to any random posts they comes across without evaluating how quality such post could be or how contributive and helpful it's to the general public before receiving such quality.