Hi Po,
a 1 1/2 inch firehose to substitute for the cheap plastic blue hose will pass the water fine. It will last you for years!
I used something else because it was there, and a little less expensive than the 1 1/2. I will post some info for you about how to connect it to your backwash pipe in a bit.
The "sand" on the skimmer. Well, its there because the water flow through the skimmer mouth isnt strong enough to take that with it. That might not actually be sand. Sand sinks rather quickly.
But it very well might be dead algae that you are finally getting the remaining bits of. If you touch it and it desintigrates, its dead algae. (dead algae looks exactly like that). If indeed it is algae, once its gone, you may never see anything like that there again.
If its sand and you touch it, well .. it will feel like sand!
lets find out what it is first before trying anything with your pump or circulation.
Regarding the FC.. dont know about you, but here where I am 100 miles south, we havent had a lot of sunlight. When its perpetually cloudy as it has been, the FC will run a point or 2 higher than normal, because the sunlight is not burning it off.
As spring and summer become more "normal" with lots of sunshine, you will likely have to adjust your FC back up a little.