The EC 30, blended at the appropriate 1-to-6 ratio, won't significantly affect the cold cranking viscosity of the oil. If a 0W-20 is borderline 0W-xx (ie: 6,000-6,200 cP in CCS), it might raise it to just inside 5W-xx range, but the impact would be relatively insignificant in the real world. There's no restriction as to the time of year. Use when convenient for you.
EC uses an HDMO add pack that isn't overly unique in its additive chemistry. Thus, it should blend sufficiently with either of those oils. If it was me though, I'd use ESP, but that's just me holding the Euro ACEA specs in higher regard than the API specs.