The Oppo does have excellent video processing, so you would not lose anything by going directly to the projector. I don't think any 4K processor/receiver would be better in this case. The only thing that could be better than Oppo video processing is if you put in a Lumagen before the projector to do the processing and color correction.
You could certainly connect the 7.1 analog output from the Oppo to the 7.1 input on the HTS 7.1. At this point, the Krell becomes a pure analog preamp (it won't mess with any analog-to-digital-to-analog conversion and you'd get the full resolution of any hi-res audio from the Oppo). This would give you pretty good resolution. Just make sure the 7.1 is in full working condition. And keep in mind that the reliability of this older product may not be long.
The resolution of the KRC 3 is not going to be as good as the HTS 7.1. If you went this choice, I would highly recommend having the KRC 3 recapped - using best quality capacitors available. The KRC series was warmer with generally less resolution, as well as the KAV 500 amp.
The only reason to use a 2-channel preamp is if you didn't care about the quality of HT at all (in other words, HT is just something to watch movies on and takes much lower priority for sound quality). In this case, you could use the left/right out of your Denon receiver to use on an input to the KRC 3 in pass-through mode. Then use Denon receiver to power all other channels.