the your NAS diplexer solution save a hell of a lot more than that and not require use of RG11? I'm guessing the reason he said he had to use RG6 to the main house was because of the conduit he mentioned. He may be concerned about successfully pulling RG11 through it, either due to lack of room or some tight bends. There's no need for RG11 for SWM, since you always have the option of amplifying the signal, and SWM amps are pretty cheap. The savings from using RG6 instead of RG11 may even pay for the amps, but it sounds like he believes he has no choice in the matter anyway.
I was assuming use of the diplexers to connect the two SWM legs in the main house, not at the SWM16 in the guest house. That way DECA doesn't have to make the long journey there and back, and there's no need to use RG11 to reduce losses due to the inability to amplify the DECA signals. I still think ethernet only is the way to go for whole home if at all possible. If he's fortunate enough that the house already has cat5 run to all the receiver locations, it is a no brainer. A gigabit switch to connect them all, and it will be a far faster and far more reliable network than DECA.
What I'm thinking for your diplexer solution is putting them inline (backwards) where the RG6 enters the main house, with the SAT outputs going back to the guest house, and the ANT outputs connected together on a splitter. The diplexers would introduce very little additional loss for the SWM signal, the DECA stays in the house, and if he needs SWM amps it won't present a problem for the DECA.
Are you sure he'd need an amp for the signal from the dish, even if he uses RG11? What's the typical output of a legacy LNB and how much loss is there in 200 feet of RG11? Are you thinking the losses will put the signal below the threshold where the SWM16's AGC would bring it back to nominal? Of course, he did say it was "200+" feet, so this may once again depend on what the value of '+' is
Diplexer option has about 1 dB loss, where the crossover of the -16 has about 6 dB.
You asked a good question a while back, but it hasn't been addressed [yet].
One of the questions might be is the connected home [aka MRV] wanted between the guest and main house?
Would there ever be an interest in having a Genie client in the guest house, with the Genie in the main house?
"If no, then" diplexers at the main house keeps the DECA losses within range.
"If yes, then":
RG11 really should be used for the 200' runs. as the loss is close to what "only" 135' of RG6 is.
If the RG6 is a fixed item, then an amp can be used between the -16's output and the diplexer [going to the 4-way].
This configuration amplifies the SWiM signals on the long runs with the 4-way splitter, while still maintaining the DECA/coax networking.
It's pushing this limit, but should be 5 dB less than the max, so
As for the loss/gain of the dish to SWiM-16:
You don't want to spec/build a system that only works on a sunny day.
You can have 5 maybe even 10 dB of loss between the LNB & SWiM without problems, but after that, with the long SWiM runs here, the SWiM AGC may not support the full range anymore.
Solid Copper Core RG11 @ 200' would have about 13 dB loss at the high end. This can be compensated with an amp with slop correction, which basically takes this distance/length "out of the loop", as it would be as if the SWiM-16 was mounted at the dish.