Maybe this has been answered before, but I couldn't find it. If a unit leader moves, but can't reach another unit it wants to engage with, can the player place the other minis, in cohesion, with the target unit, thus initiating engagement and moving all of the minis including the unit leader? Pg. 32 of the rrg says unit leader, but it's not clear to my dumb brain.

Also, can this be done with force push? E.g. Vader force pushes a unit leader towards him, but not close enough to touch, then moves the rest of the unit in cohesion to make base contact and engage?

Regarding your first question, you cannot initiate a melee with a mini that is not a unit leader. Apart from that rule you mention from page 32, there is this rule in page 18:
"When placing a mini in cohesion, it cannot be placed in base contact with enemy minis, unless the unit is in a melee (see “Melee” on page 32)."

For your second question, yes, Vader (or Luke) can use Force Push to move a unit leader to base contact with him to start a melee.

Point 2. Its the same as point 1. You can force a leader in to melee and therefor the unit, but if force push cant make the leader enter melee then the rest of the unit also can not enter melee. Force Push is a standard move in every way.