Originally the Avantiair lease was up until the end of April when the Jump Air ATR came in, so I presume they stayed a little longer to help cover the Luxwing.
According to what I read last week (can't remember which of the local news outlets), the Embraer is limited on how long it can fly between now and when she goes due to needing maintenance which is why despite delays/ cancellations she isn't being used much and G-ORAI was due back by 10 May.