Is it okay to swirl agar in the petri dishes?

In a few words, saving LB agar is not worth the trouble. Even not considering the potential contamination issues (that depends on how well you work), the plates tend to dry when left overnight at 37�C, and if the plates are not full enough, the medium will dry too much and the colonies will grow too small to be of any use. So, fill the plates generously with LB (you must avoid the LB to touch the lid though) and it'll be ok. I would discard this batch of shallow plates.