wow.. lots of Q's in that lot!

Point form in no particular order

- the only process I've heard of (never done it myself) to purposely turn negs into slides is for B&W. You can cross-process E6 & C41 but I don't think what you end up with is a true reversal.. just weird colours.
- Kodachromes can require more post scan work to get accurate colours if you're scanner can't chew thru the density (like mine!) I'd imagine a decent film scanner would breeze though Kodachromes these days.
- Negs scan fine. I wouldn't be printing colour negs just to scan them.
- Prints can be scanned fine, but you need to be aware of final resolution required. Scanning a 6x4 print at 'lots of dpi' to get the file size uo doesn't always work well, you end up getting chararteristics of the paper into the scan.

I think you can scan from whatever the source you have material you want to use in. For stuff you've already got you don't have a choice! You need to decide whether the benefits neg film gives (less fussy about exposure, harder to colour balance, cheap to buy and get processed without prints) are more favourable over the benefits of slides (accurate colour if exposed correctly, better scanner required usually).