Good stuff, great questions, you are catching onto this thing, will be an expert by the time your done. I agree with Mr. Bruce
The reason (I believe, or the reason I was going to say check your gauge) is because a backwash should decrease your pressure not increase it, because you just got rid of all the dirt in your filter the water runs through with less resistance.. So something not quite right. However I have had that happen a few times, when it did, I would turn pump back off wait a minute and turn back on and it would always go back to 15 (my clean filter pressure). I always though because it was a cheap gauge, I think I may take Mr. Bruce's advice and buy one from TFTestkit.com. They have a special promotion going on, free shipping on orders over $70. Running low on reagents, they have best prices for volume I have found, and I usually get my order in 2 days. (no, I have no financial interest).
all the returns flow with more pressure
OK this can make sense because less dirt in filter, more water getting through.
I'm thinking that it's more important for the return in the deep end to have an increased flow than the shallow ones. A slight turn of the valve and the flow changes by a lot.
The valve should almost have a detente when they seat properly in a position, maybe a slight dirt back up at valve. I believe I read Swampwoman say to move the valve back and forth a little with pump on to try and clear, but wait til she chimes in, I PMed her and asked to to look into this thread if she has time.
I would like to see even flow on all, but I do see some logic in having more flow in deep end.
I'll find out more info about the heater but it looks like the PVC pipes coming from the heater, lead to the returns. It makes sense to me, so the heater pushes out warm water through the returns. Even more important that the deep return is back in play.
Great idea, In the plumbing I usually see, there is a valve to bring the heater online (bring water from pump through heater), then water coming out of pump into the main return line. But yours could be different and it is very important to know exactly what your system is and does so you can troubleshoot. Inside the heater where the heat is transferred to the water it is usually copper piping.
The owner of the pool store (not the knowledgeable guy who helped me) said that if I turn off the filter for 24 hours, I may be able to see the bottom of the pool, to see how much debris is left.
Not necessarily a bad idea in my opinion, If you turn off pump the circulation drops and some fine particles may fall to bottom and you may have more visibility, then be able to vacuum out. Yes it does interfere with SLAM a little, but not too much. I was actually going to suggest this but I figured I would wait a day or 2 to see if Swampwoman has time to get in here and comment. If you were early on in slam I would not advise it, but may be helpful at this point. My suggestion would be to do it at night not 24 hours.