1. If you try the Disengage maneuver and fail, do you still manage to back up and break off from melee combat (incurring a free attack by your opponents)

2. Yes

1. The Disengage Combat Action is a Full Action and requires the character has somewhere to move to (by moving up to AB metres), that isn't in melee range of whomever the character is already fighting in melee. But apart from that, it isn't something a character can fail to do. There's no dice rolling involved.

Jargal already answered your second question

EDIT: On second thought, you can reduce Disengage to a Half Action by passing an Acrobatics test. However, failing that test simply makes Disengage work as normal - that is, it is a Full Action.

It's worth pointing out that you can also split the Aim action. So on turn 1 half move half aim, then on turn 2 half aim and shoot with a +20 bonus for Full action aiming.