I think we are at the endgame here.

First, the answers to your questions. The backwash plumbing is one 90 degree elblow and 10 ft. of 1.5 inch flex. The pressure gauge does go to zero when the pump shuts off (and was replaced, see below).

The most recent diagnostics and changes:

I completely replaced the (Tagelus) multiport valve. (I disassembled the valve, checking for blockage. There wasn't any blockage, but there were small rocks (!) embedded in the (non-replaceable) spider gasket. It was cheaper to replace the whole valve than replace just the diverter.)

I installed the new multiport valve first with the old pressure gauge and then with a new pressure gauge. There were only minor differences in pressure readings between the old and new multiport valve and the old and new pressure gauge.

I temporarily removed the flapper from the Jandy check valve, but there was no change in backpressure.

Reminders: All piping is 1.5 inch. All valves are 2 inch valves with reducing couplers. Pump is a 2-speed, 1.5 hp Hayward Super Pump. Problem is that at low speed, with the heat pump in circuit, there is not enough pressure/head to operate the water feature or the chlorinator. The water feature and chlorinator will function at low speed with the heat pump on bypass. The water feature and chlorinator work fine with the pump on high speed and the heat pump in circuit.

Summary of backpressure readings:

To filter, to waste - 8-9 lbs.
To filter, to rinse - 14-15 lbs.
Through filter, to backwash - 15-16 lbs.
Through filter, Nature2 (empty), check valve, chlorinator, pool returns (full) + water feature (full) - 18 lbs.
Through filter, heat pump (incl. 60 ft. of pipe out and back), Nature2 (empty), check valve, chlorinator, pool returns (full) + water feature (partial) - 23-24 lbs.

There doesn't appear to be anything "wrong" with any particular component of our system. The addition of the heat pump and extra piping made the performance of the system marginal last year and, for unknown reasons, sub-marginal this year.

I believe I have three options:

Status Quo - I rewired the (mechanical) controller so that the timer turns the high speed pump on and off. There is no upfit cost, but there is probably extra cost for electricity. I could run the calculations and see if I could operate the pump for less than the current 12 hours a day.

Replace all above ground piping with 2 inch pipe. Benefit would be somewhat reduced since the underground return to the pool is a single 1.5 inch pipe and the underground water feature line is a 1.5 inch pipe whose valve is only partially opened.

Replace the Hayward Super Pump with a new pump, probably variable speed/flow or high-efficiency. Most of these new pumps require 2 inch piping. I could upfit the piping to 2 inches and then, if still necessary, replace the pump now or later.

Do you have any other options to suggest? What would be your recommendation or ranking among the options?

Thanks again.