I asked my Dr if the oestrogen could still work even after starting the progesterone and he said yes - it changes appearance to become secretory and loses its triple line but the oestrogen can still exert an influence
It obviously did as I started the progesterone the day after EC and my lining had grown 3mm even after it had started to compact.
My Dr said he would not risk transferring a good quality embryo into a suboptimal uterine environment and if there was any doubt about the endometrium we would freeze all
At my lining scan he was very pleased with the response and said he was confident it was reasonable to proceed as things stood
The concern was the unpredictable nature of my lining - that it had thinned from 7.2mm the day before trigger to 5.5mm at EC.
So I kept up the oestrogen and the rest, as they say, is history
At my viability scan the lining was appropriately thickened and appeared to be supporting the pregnancy beautifully
So with respect to Dr Sher, absolutes aren't helpful. It sounds like you are in terrific hands and they won't recommend transferring unless they're happy it's worthwhile to do so.
Thinking thick!!