For the argument concerning moor management and flooding I think I would want to see a detailed study over time of river velocity and height against rain gauge data, flooding events and moor management.
It may have some merit but I would want to see a detailed study before making any assumptions, it superficially seems more another means to attack grouse shooting, though I could be wrong..
I suspect intense bursts of rainfall cause the flooding events. Burning heather has been an upland management process for many many years. As the heather gets older, higher and more woody the nutritional value and attractiveness to grazing animals decreases. This is not just an issue for grouse moors, it is valid for all moors.

https://en.wikipedia.org/wiki/2015%E2%80%9316_Great_Britain_and_Ireland_floods
https:
//www.theguardian.com/environment/2016/sep/08/flooding-uk-government-plans-for-more-extreme-rainfall

http://slowtheflow.net/wp-content/uploads/2017/02/HB_Dissertation_A-Clark-Final.pdf