Is it no longer possible to self build hours?
Yes, its still the primary route and is likely to continue that way.
I could then fly RHS in that jet until I have done 500hrs in the RHS, and then my ATPL becomes "unfrozen" and I can move over to the LHS.
Nothing ever becomes "unfrozen" If you have met all of the requirements for ATPL issue then you can apply for the issue of a New Licence otherwise, you still have a CPL/IR!
Whereas the JAA 500hrs multi crew time (needed for an airline job on a European carrier) is totally beyond what almost anybody can achieve on their own.
It could be argued that unless you have 500 hours RHS time you don't need an ATPL!