Should I be waiting till after all risk of frost has passed?
Middle of May might be reasonably expected to be frost free, but not certain by any means. I have seen it snowing in June at my location.
Some common sense required here. If bees are not clustered, any incursion by mice will be defended.
I cannot really understand the lack of movement away from mouse guards now OMFs are the favoured floor type. It only needs an entrance of one bee height, not 22mm. I do not make any other defence on my hives with the minimum height entrances.