Does the Macho Brace reduce speed

The participating Pokemon only get experience once. Everybody else in the party gets half of the total each.

Experience gain is independent of participants and number of Pokemon in your party, so yes, your party gains more experience overall if you have more people in your party at the same time.

All pokemon earn experience independently. So the rest of your party does not affect the amount of experience they earn. This is also independent of the Exp. Share's on-state.

I do not know how EV's through Battle work in Gen 6. So I won't comment on their distribution here.

So five example Pancham Battles:

Battle 1a (4 party members):

• Pokemon A participates.
• Pokemon A earned 80 experience.
• Pokemon B,C and D earn 40 experience each.

Battle 1b (3 Party Members):

• Pokemon A participates.
• Pokemon A earned 80 experience.
• Pokemon B and C earn 40 experience each.

Battle Example 2a (Exp. Share On):

• Pokemon A and B participate.
• Pokemon A and B get 80 experience each.
• Pokemon C and D earn 40 experience each.

Battle Example 2b (Exp. Share Off):

• Pokemon A and B participate.
• Pokemon A and B get 80 experience each.
• Pokemon C and D get 0 experience each.

Battle Example 2c (3 party Member Exp. Share Off):

• Pokemon A participates.
• Pokemon A gets 80 experience each.
• Pokemon B and C get 0 experience each.

Traded Pokemon do get boosted experience even if they do not participate.

This is apparently a roller blading technique. There is an NPC in Lumiose city who tells you that if you move the analog stick in small circles in the direction of movement, you will use a special technique.