1. The requirements for reverse transcription of HIV and other retroviruses are presented in the post generally (specifically, the requirement of a primer that is physically associated with its RNA). I would invite you to read it.

Any discussion of endogenization is entirely academic given that the mRNA has no means by which it can enter the nucleus once inside the cytosol. Gene therapies using mRNA for example serve to either replace a hypomorphism by supplying the pertinent cells with the appropriate gene product or function to suppress the synthesis of a target protein via antisense oligonucleotides, both of which are processes that occur in the cytosol.

Furthermore, the endogenization of a sequence containing a spike RBD which the immune system has already been primed against, given in the form of a parenteral injection, as discussed in the post, would affect a small number of cells that, upon presenting the epitopes of the spike RBD, would be rapidly cleared by the relevant immunological effectors.

I was not specifically asked about other RNA viruses which can integrate with the host's genome, so I elected not to, as the post was already more complicated than I wanted it given the intended audience.

2. I have an entire separate post discussing the normal fate of exogenous RNA once inside the cell and the numerous degradative pathways it can undergo. My aim here was to answer explicitly whether or not it was theoretically feasible for mRNA vaccines to be able to somehow alter host DNA and I was supplied with a series of increasingly unlikely scenarios to consider.