1. The BDP-105 is only $1200 and if you are paying more than what MonoPrice charges for RCA cables then you're wasting money on something that provides no audible difference. Subjective looks of the cables are another story.
2. If you plug the Oppo into the 4520 into one of it's analog inputs other than the multi-channel inputs then you are no longer using the Oppo's DACs. You are now using the 4520's DACs and would still be able to take advantage of Audyssey.
3. If you connect the Oppo to the MC inputs of the 4520 this will bypass Audyssey automatically. No need to put the receiver in to Direct or Pure Direct mode.
4. Yes, you will lose the convenience of Airplay if you are playing through the Oppo. You will either need to directly connect the device as a USB device or stream it via DLNA.
Will you hear an improvement going from the 4520's DACS to the Oppo's? That will all depend on your room and gear. If your room is well treated to the point that Audyssey has very little to do to correct frequency response then you may very well hear an improvement. I doubt that it will be huge though. How much of that difference is worth to you will be solely up to you and your wallet. All I can tell you is from my experience with the BDP-93 and BDP-95.
My room is carpeted and has acoustic panels on all four walls at the first reflection points as well as diffusers on the ceiling at the first reflection points for the front three channels. It is completely sealed and ~2300ft^3. I have a Denon AVR-4311, Emotiva XPR-5 for the front five speaker channels, 2x TC Sounds LMS Ultra 5400 18" subs powered by a 14kW amp and my speakers are Revel Performa2 F52 for the front pair. I have both the Oppo BDP-93 and the BDP-95.
I have the BDP-93 connected both via HDMI and analog to the MC inputs of the 4311. Here is the order of my preference from worst to best:
1. HDMI with Audyssey disabled and speakers set to small with sub enabled. Overall sound and staging is good, but a little muddy and bass a little bloated.
2. HDMI with Audyssey enabled and speakers set to large with sub disabled. Huge improvement from above. Sounds considerable cleaner especially in the bass regions.
3. (For movies and bass heavy 2CH music) HDMI with Audyssey enabled and speakers set to small with sub enabled. Another big improvement in a more full sound. The Revels just don't play with authority in the lower bass regions. The subs help tremendously in that respect.
3. (For critical 2CH and MC music) Analog to MC inputs on the 4311. Running analog is only a minor improvement over HDMI with Audyssey. Clarity is the same, but has a warmer sound to it. Bass is the same and well controlled as opposed to HDMI with no Audyssey. Doesn't make sense how that would be, but it is what it is.
The BDP-95 improves on the BDP-93 only slightly. For movies I'll still run HDMI with Audyssey as that has more flexibility in terms of control over the subs. Also, I'm not too concerned about absolute audio quality when it comes to movies.
In short, I think room treatments would make a much bigger performance increase than going from the DACs on the 4520 with Audyssey to the DACs on the BDP-105 will. You may actually get worse performance out of the Oppo DACs if your room is not well treated.
One thing to keep in mind is that these are all based on the 4311 and not the 4520. The 4520 may have different setups for their analog inputs vs. the MC inputs. The MC inputs on the 4520 may use the internal DACs also, but I don't know the answer to that either way.