Is this a current mirror?

Q55 and Q56 are transistors that have multiple collector terminals, where one of the collectors is shorted directly to the base contact.

This keeps the transistor from fully saturating, similar to how a Schottky transistor works in 74S and 74LS type logic. Keeping the transistors out of saturation improves the performance of the circuit.

Not current mirrors. An example of a current mirror is Q60 and Q61.

Yes.

Hans Camerzind writes in Designing Analog Chips, on pg 3.3:

There is also a lateral PNP equivalent. Using a split collector this current mirror needs only a single device. Each collector being smaller, the maximum current is limited (100uA)

So this is a smaller way to implement a high side current mirror.

On the high side lateral PNPs can have 2 or more split collectors and on the low side, you can have multiple identical emitters on the same base. Both these can be used to give current mirrors with current ratios of 1:N