The PoD would be before the battle of Selasia, with Cleomenes either assuming control of the aechean leauge's military forces as general, or usurping control of the league by inciting revolts amongs the lower classes. In both cases, the macedonians would have to intervene on their own, instead...
The most critical POD for Cleomenes would have been his OTL demand to the Achaean Commonwealth to be elected general by the leauge assembly, after having thrashed their armies at the field. His demand was initially accepted by the achaean oligarchy, but Aratos procrastinated the final decision...