Yes it's true that the Oppo is basically a self-contained HT processor. However, it's great that you caught that "single HDMI input" limitation. I was going to mention that.
The Outlaw does not have bi-amping. I wouldn't normally bi-amp unless you had an out-board analog crossover. That's really the only true benefit to bi-amping (but you would need to modify internal speaker crossovers). Also, bi-amping in processors can frequently degrade sound quality because of the way they automatically try to do bi-amping signal differences.
You could always do a Y-Cable, but then you are affecting the amplifier input impedance. The Monolith has more than enough power to drive your B&W speakers without bi-amping.
As far as the 5-channel vs. 7-channel. The 7-channel has bigger transformers, so it's possible that it would give you slightly more power in the bass/midbass area, especially if you don't connect the last 2 channels. It's up to you. It's basically a $230 price option, so it depends on your budget.
I don't know if you noticed, but I typoed the price for the 5-channel. It should be $1500. NOT $5100, lol.
One thing about the Outlaw is that the menu/setup is somewhat cumbersome. It takes about 3-5 seconds to bring up the menu and it blanks out the audio/video. It pretty much treats the menu system as a completely separate source. It doesn't have OSD overlay like every other processor. But you can configure it and the performance is quite good.