OK, not having BH's on the bottom & still getting Detritus up & over so it can drain into the sump. MY thoughts only so don't bite me
I don't see any more danger in having a BH on the bottom as compared to 4" off the back bottom, you get a leak basically your going to loose almost all of your water. I did use sch. 80 BH's on all the fittings because they are so beefy & just seem to hold up better, I once split the lock-nut on a regular bh, so that sorta made me shy of using them, bit the bullet & got the bigger & thicker BH's just to feel a slight bit better about that.
With the header system I have on my tank & the MBV (which the OM could do the same with slight more plumbing) I reverse flow every 5 minutes during the day & 10 minutes at night. I loose no momentum as Mark mentioned, granted my tank is only 4ft long but MoJO uses the same process just more pump & has no issues with that. I have two spots the collect LR shed but detritus ends up over the coast to coast trough, this trough is very key to skimming the water & getting detritus out of the main display, you can't do that with two corner overflows, they will not collect nearly as much. I don't see an advantage of having bottom BH's in the rear of your tank as in your drawing, you could move the back outlets to the back wall & get the same directional flow using loc-lines. This would allow for more bottom area for LR etc, no chances of hitting the bottom BH's causing possible damage etc! I would have two outlets on each side that would get water together to make the wave motion from that direction, blowing everything to the other side like one wave, then when the OM switches, they go off & the outlets from the opposite side would blow across in the opposite direction, each outlet with "Y's, so that would equal to 4 outlets on each back side blowing across the tank & two up front swirling the water back, that would be more than plenty of flow, also adding to that whatever returns you have from the sump can be up high. Also all of your flow won't be coming from the bottom up but rather midways across, sorta side stepping. Last thoughts, on the outlets split each CL outlets with loc-line "Y's" & split up the flow even more with wide flare nozzles. With this said I have smooth operation & I can determine the switching time as desired, I get flow everywhere & at the same time no focused streams of water squirting out rather a wide smooth flow, I thought that maybe I would need a Tunze stream or two opposing but no doubt I plainly don't need any, nothing, notta. The only way for you to really appreciate it is seeing it.
oh just had one more thing, even the outlets up front isn't necessary IMO! Why? Well, across 25" I get water from that wide spread &
8 outlets on EACH side (4BH's & then "Y,s" makes the 8), some of that water hits the front & bounces back, just like you would expect, this then bounces toward the other side because of the angles, even through LR I get flow coming from the front.
Just some thoughts to consider