The reason they got immunity is that the percentage of claims about vaccines that are valid are minuscule. Juries don't do well when faced with an obviously harmed "victim" and a "bad guy" with deep pockets. They see the suffering victim and no evidence to prove yes or no on whether the "bad guy" inflicted the harm and tend to side with the victim.

The only way the vaccine industry can survive in such a legal climate is to get the claims reviewed by medical people rather than juries. Lawyers hate it because they can't woo the jury with the plight of the victim.

Society would work a lot better if more systems worked this way. Locally the lawyers utterly hated the system where to bring a malpractice case you had to go before a board of three lawyers and three doctors first. While you could then go to trial with less than 4 votes from that board your chances were minimal when the jury heard what the board said. Effectively that meant you had to convince one of three doctors your claim was sound--if you can't do that why do you think you have a case?

mRNA can be used as indirect vaccine, by manipulating cellular factories to produce a foreign object that draws the attention of the immune system.

There is no known pathway of persistence in this mechanism, and there is good reason to believe such a mechanism cannot exist as it would be a highly versatile attack vector and viruses would have evolved to be much more virulent by using it. Retroviruses do complex work to infiltrate the DNA repository of a cell, if all it took is to drop some mRNA it would have been exploited and 'patched' a billion years ago.