According to the official Disney list of Mickey Mouse Shorts in the book "Mickey Mouse, the Evolution, the Legend, the Phenomenon" (published by Disney Editions) there is already 19 Mickey Mouse shorts outstanding - so no need for the film segments to 'bump up the numbers' Being as this is a Disney book, we can only assume that this is Disney's official list of Mickey Mouse shorts.
1. Society Dog Show
2. The Pointer
3. Tugboat Mickey
4. Pluto's Dream House
5. Mr Mouse Takes A Trip
6. The Little Whirlwind
7. The Nifty Nineties
8. Orphan's Benefit (remake)
9. Mickey's Birthday Party
10. Symphony Hour
11. Mickey's Delayed Date
12. Mickey Down Under
13. Mickey and the Seal
14. Plutopia [*]
15. R'Coon Dawg
16. Pluto's Party [*]
17. Pluto's Christmas Tree
18. The Simple Things
19. Mickey's Christmas Carol
20. The Prince and the Pauper
21. Runaway Brain
[*] I understand most people consider these as Pluto Cartoons, which is why they are left of most lists for the second Colour Mickey set on the Internet. However Disney appears to consider these as Mickey cartoons. Other Pluto cartoons which star Mickey (such as Pueblo Pluto and Squatter's Rights) are not included on their list.
So I personally think the above listed cartoons will be the ones included in the set.
The extended runtime will include the supplements as well. I speculate that being as this is (chronologically) the last Mickey Mouse set - and dealing with more recent material, the supplements will be more extensive than any other Treasures set to date. I still think the film segments will only be discussed (with a few clips shown) rather then included complete in the set.
And hey, that Mickey Mouse Disco Easter Egg (should it be included) runs for 1/2 plus doesn't it? 
Edited to change the order of two of the shorts on the list.