1. I would prefer two separate drain lines. My 120 has two lines and they are each plumbed directly to the sump. Less issues if there is a clog etc. Adding the option for an “emergency” valve to divert water is a “nice to have” I suppose. You could also just pump out of the sump to the drain.
You have the pipe lengths right but did you add any for fittings? By my calcs you may be pushing 6-800 gph if they ran output of one pump into input of the second pump to boost your head pressure rating on pump. Two drains into one drain is begging for a flood. I have more drain and return on my 125. I would first contact the builder of this system, send them all your concerns and complaints. If they can't remedy these then either sue or dispute claim with credit card. Stand for sump trash that thing build a stand big enough to set both sumps on at work height have them drain into sump at the back wall. You could mount a pump right below tank( cut out a 90 add external pump have lower head pressure pumps in sump pump to pump to tank). Rip all those valves out I won't say get the BRS valves but home Depot and Lowe's both sell a double union ball valve worth the buy. Some reference check valves personal opinion waste of money and point of failure with the sumps you have you should have plenty of room for volume. Again personal opinion run one sump with skimmer and one with filter roller and fuge( like right side sump skimmer, any reactors, one heater one return pump. Left side drain into filter roller into fuge heater number 2 pump number 2).
