Formigli et al, in 1987 reported uterine lavage of embryos from uteri of donors at 5 days post-ovulation. The embryos were then transferred to recipient women. They had pregnancies when the recipient's cycle was from 4 days in front of to 3 days behind the donor's at ovulation. This suggests a window of implantation of up to 7 days. Pregnancies resulted from transfers on all days.
Blastocysts can hatch and implant.
The HCG assay used can detect very low levels. They did not find any evidence to support the concept of an embryonic diapause in humans. Success rates for frozen embryo transfer cycles vary considerably by the program handling the case. Checking the SART results for that program for frozen embryo transfers. Asking very specific questions of your physician there.