My two cents, as an audiophile with plans to upgrade my 718's base stereo:
1. Haven't, and won't. My big concern is sample size; there's no info in the Owner's Manual regarding what bitrate the Jukebox stores digital files in. My previous car (BMW 2 Series) actually downsampled to 256Kbps to maximize space. If 256 Kbps sounds fine to you, great, but it doesn't to me, particularly considering the bass response that even the base Sound Package Plus audio has.
2. This has been answered. However, the two SD card slots can take up to a 128GB SD card each. That means 256GB of media storage (audio/video/etc.) in nearly any format you choose. The kicker is that the system will read files in nearly any lossless codec except for AIFF and OGG. I already have a 128GB SD card stuffed with FLAC audio files in my car; playback is great. That makes the Jukebox trivial, IMHO.
3. No. It's almost certainly a partition on the PCM's HDD.
4. Haven't tried it. That said, other feedback suggests it's reasonably quick, and so far I've been impressed with how quickly PCM operates. PCM4's got a nice-sized processor in it. Great for us!