blaice said:

QUERY 2:
Is there any upside to directly muxing the 17.8gb MKV to BD ISO and putting the resulting directory on a blank BD rather than getting the reduced AVCHD for DVD? The drop in size from 18gb to 8gb seems intense but I have little esoteric knowledge in this area so I thought I'd ask. (Is it similar to the audiophile argument between FLAC and 320kbps when most find no discernible difference etc?)

The bigger encode should look somewhat better, but its not a huge difference. The other difference between the two is lossless audio and foreign dubs. If you want those, burn a BD. If not, the AVCHD is fine (and is ready to burn.)