Spa Removal & required piping and equipment modifications

[Spike]

We have a 10,000 gallon inground pool with an attached spa built in 1986 or so. The spa equipment was removed some time ago and we are planning to remove the wall between the pool and spa. The pool pump and filter (installed many years ago by a local dealer) don't seem to be working very well for us. The pump is 1 hp and is pumping about 50 gpm with 2" inlets and 1.5" discharge piping. The water is circling inside the pool and moving so fast it frequently tips over the pool cleaner (Harward pool vac).The piping we have been able to dig up (not under the deck) is pvc 1120 (200 psi) (unfortunately not schedule 40). The filter is a Hayward DE 48-20 and seems to be massively oversized but that may not be a problem? (here near Phoenix we have occasional dust storms which can drop a lot of dust and debris in the pool). There is a multi-port valve on the filter (which may or may not be completely functional). We can backflush and normally run the pool buy sometimes when the pool is shut off DE/dirt/whatever is in the filter leaks back to the pool/spa. The burred discharge line for the spa had a longitudinal split along the top of the pipe for a length of about 6', obviously causing a substantial leak anytime there was water in the spa.

Questions:
1. Can I permanently cap all the spa pipes underground with the understanding that when we refinish the pool and remove the wall between the pool and spa all spa inlets/outlets will be solid grouted (they are currently plugged)?
2. Is there a backflow function on the multi-port valve (which has failed) or why are we getting DE flowing back into the pool? Should there be a check valve somewhere on the supply side of the filter?
3. With discharge pipe velocities in excess of 10 fps is it appropriate to assume that surge from many years of pump cycles caused the pipe failure?
4. The pool is currently not leaking. Can we make modifications to protect the remainder of the underground piping or is it time to replace the pump and substantially reduce the flow rate? Is there a minimum required flow rate to make sure the pool cleaner will work properly?
5. We also have optional discharge pipes to pipes thru a planter and a spray nozzle to cool the pool. Is there a neat and easy way to split (or shut off) the flow to these other 2 discharge pipes or do we need the extra 5 -90degree elbows and 2 vertical discharge pipes with ball valves provided in the current piping design?

Before anyone tells me to call a professional, please understand that the current pump (probably double the capacity we should have- likely caused the spa pipe failure) and the filter rated for about 90 gpm (twice what we have and probably 4 times what we need) were both selected and installed by local pool "professionals". The names of good pool professionals in the Phoenix area would however be appreciated. Many thanks for any help you can provide!

[Teapot1]

1. Yes you should be able to do that. Without a schematic thats the best I can say.
2. Usually there is a backwash position in the multiport but it shouldn't cause the problem of DE getting into the pool. With your suggested flow rate you were not really filtering as breakthrough would have occurred. If you are remodeling may I suggest getting rid of DE, its expensive, messy and nasty stuff if it gets into your airways. A sand filter with Dr Drydens AFM ng filter media does a better job for less (with the correct flow rate).
3. How do you know you are getting return discharge flow of 10ft/second?
4. You could fit a pump speed controller, that will reduce the pump speed and flow, save you a fortune in electricity and improve filtration and water quality, what pump do you have?
5. Not something I have seen in 22 years of being a pro, OK we never stop learning but sounds like you could do without it. Schematic needed and a few pictures may help.