1) Is it correct that these have a 10 year validity?
2) Would a legal extension to the house change the EPC validity, for instance a conservatory with a large window area?
Generally, no. This official document states (on p18-19) that:
An internal refit with new heating, hot water, air conditioning or mechanical ventilation etc., would not trigger the requirement for an EPC, unless the building were also converted so as to comprise more or fewer parts for separate use1. Any refit will, however, be subject to such of the Building Regulations as are applicable to the work.
3) Can the EPCs retrievable from the EPC Register be used legally for a sale?
Both the register website, and the relevant page from the government website, appear to suggest that yes, you can.
Footnote 1 (from the same document):
A part of a building designed or altered to be used separately is where the
accommodation is made or adapted for separate occupation.