In view of this change can the NPPL + PMD be used on an N-reg?
Yes for the licence. It is now rendered valid under the UK Aircrew Regulation so satisfies art 148(3)(c) of the ANO.
The PMD should be acceptable to the extent, if at all, that it satisfies 14 CFR 61.3(c)(2)(x): “Is operating an aircraft within a foreign country using a pilot license issued by that country and possesses evidence of current medical qualification for that license”.
Does this also mean that a UK PPL with a valid SEP rating and a PMD can be used in an N-reg in the UK after 21st December for a UK Resident?
It can be used right away if the PMD satisfies the above US regulation.
It may satisfy the US Regulation, but it doesn’t look like it satisfies the UK requirement to convert to a UK Part-FCL licence (with presumably a full medical rather than a PMD) :
That’s an exemption for US pilot certificates. The Aircrew Regulation amendment (SI 2021 No. 614) is for UK non–Part-FCL licences.
