If the only kohen enters the shul on yom tuv after davening. can he do birkas kohanim with a beracha because it is the zeman of the beracha or it has to be during davening to make a beracha?
Birkas Kohanim can only be done during the actual davening, and it cannot be afterwards. The reason is because Birkas Kohanim is meant to be attached to the bringing of korbanos and avodah. This is learned from the verse that Aharon Hakohen said Birkas Kohanim after he brought the correct korbanos. This is why the kohanim have to go up during the bracha of retzei, when we daven that Hashem return the avodah of the kohanim to us. This applies to saying Birkas Kohanim with or without a bracha.
Sotah 38b, O:CH 128-8