1. Can an SSEA rating be signed off on completion of a LAPL proficiency check (both have similar if not identical requirements)?
Yes. There is no sign off for a Prof Check for a LAPL , it is a Log Book entry. They are both UK licences and doing the check can result in renewing the NPPL and validating the LAPL.
2. We do not yet know the NPPL (SSEA) or LAPL (SEP) route to what will now be the full fat UK PPL
The CAA made it quite clear on their Webinars a couple of weeks ago that nothing is going to change in the UK for the next two years. The upgrade to a UK Regulation Licence is exactly the same as it is now. Obtaining an old style UK licence is more problematical as the CAA is intent on hindering this process despite it being in the ANO.
The process of adopting the EU regulation into UK Law is more likeley to be a shot in the foot than a panacia. The LAPL will not be acceptable by the EU so is a rather pointless licence in the UK but converting the NPPL to a PPL is still going to be subject to the rules they are adopting on 31st December.