Ubiquiti Uk-Ultra works as a mesh extension for an existing wifi. It could work as point-to-point, but one of the two Uks has to be connected to a Unifi console and would not be very efficient anyway. There are other, better performing devices for wifi bridges.

no, not to my knowledge

Ubiquity has other devices which are intended for point-to-point. Would you also prefer to buy the AC mesh (at a favourable price)?
But you can indirectly "bridge" in the Ubiquity interface by setting another AP that can use uplink or downlink...

DHCP can be set to either relay or server mode.

From memory I think it's possible to activate mesh mode from the ubiquiti console. Please check.