Why are Reptilians willing to become an imposter for a human, knowing they will have to live out that person’s relatively short remaining life lifespan? Do they literally die, sacrificing the opportunity they could have otherwise lived perhaps 100 or more years in Reptilian form, or do they somehow escape that fate?

You have seen the correct interpretation here intuitively, that they are not throwing themselves on a funeral pyre for a short time in service to their cause at the expense of their own life. Knowing their nature, you could reason this out simply because they are too selfish and savage to be wanting to serve a cause at their own expense. So what indeed happens, is that at the time of death of the physical human, their energy escapes in an interdimensional form, and then they revert to their Reptilian physical form in another location to maintain secrecy. If you think about the fact they move between dimensions routinely in carrying out objectives, this is no great feat for them although it is beyond your understanding of physics at the present time as a human. This completes the subterfuge, and so their only sacrifice is those relatively few years living out the life of a government official, for example, which may be only a few remaining decades at most. In most cases, those in power have achieved it after a long period of career advancement and it is only at the later stage of life they are given the reins of power. And so this can vary, but in most instances is not that huge a time of service.