Question 5 : Swapping floppy disk for usb would be a benefit.

#1. USB is current
#2. Instead of 15 midi files on a f.disk, you could have thousands of files on one thumb drive

Question 4. The SD1plus / SD1 is not a multitrack audio recorder

Q2. Answer is yes. You are limited with how long a sample can be. The hardest part is syncopation of the sample and song or midi file play back.