This is a rather tricky method to get used to it seems, but I am sure it is definitely doable once you get the hang of it.
I also go 50,000 score in arcade in an hour of play, but I was playing on easy. It seems to me, the only difference between hard and easy is....
1) Hard you get much, much less time to solve, so little you can likely not use a solver without using your exploit. On easy, the timer basically never goes down at all.
2) You begin on a slightly more difficult level. On easy, you start out getting 15 per level or so, while on hard you start out getting 100 or so. I am fairly the incrementation per level is fixed.
The fact that it takes the same time to get 50,000 on the different difficulties tells me the scoring is basically the same. I also was getting 1,400 per level when I broke the 50k mark.
So from what I can tell, you dont need to play on hard to get this the fastest and 50k can be achieved equally as quick with no exploits and a relaxed feeling in arcade mode.