#1 Krikkz's MSU-MD is nothing more than a custom Mode-1 driver that directly interfaces with the CD-ROM hardware, bypassing the Sega CD BIOS. There is nothing really new about it. Just taking an already established feature and packaging it a different way to fit his Mega EverDrive Pro line.
#2 MSU-MD is capable of seamless loop but it is dependent on the patch. In essence, a sound pack is married to the patch that contains specific loop points within it. A new patch would need to be made for a different sound pack.
#3 MegaSD always supported MSU-MD since day one. All that is needed is replacing the header to notify the MegaSD to force Sega-CD features for Krikkz's Mode-1 variant approach (ie, the MSU-MD driver). A future firmware is planned to remove the need for header modification.
Lastly,
MD+ is a new standard, alike to Ikari's SD2SNES MSU-1. It shares similar traits, other than just enhanced audio. MD+ is capable of streaming to/from SD card (ie, data, audio, video, etc). MSU-MD as it stands, cannot do this. MD+ features have not been fully utilized by anyone as of yet.
Note: I have heard so many bullshit stories as to why Krikkz chose to take an alternate route. This is no concern from a business stand point. What is bothersome is that so many people out there claim that MD+ is closed source. It is not, all the features are fully documented for anyone to implement.