Before my take on what possibly had happened after Mclane left Quail behind, there is some context from the story that takes precedence in order to support my thoughts. Firstly, and most importantly, Quail cannot be shot dead and disposed of as they had intended. One of the stipulations that the visitors had made clear is that if Doug dies, they will return and carry out their invasion, so execution is no longer an option. Another possible solution would be yet another memory transplant. However, the odds of an outrageously absurd fantasy scenario turning out to be legitimate recollections for a third time are just as likely as it is happening for the second time. There is a point at the end of the story where the psychiatrist mentions how nonsensical it seemed, “..this production is a grotesque dream of your childhood; it’s no wonder you fail to recall it.” Lastly, to highlight just how dangerously arrogant Quail is, the entirety of the story revolves around how important he thinks he is. Although it turns out that he is quite extraordinary, not having the memories accessible leaves him weaving in and out of this desperate state to live up to his own standards of grandeur.
The clearest and presumably safest option that the organization has is to maintain Quail’s vital functions for an indeterminate timeframe. Rekall has the recourses and the technology to implant memories into the psyche of their patrons, they certainly have the capability to keep someone in a suspended state of consciousness. This would raise little to no suspicions from anyone as we are not introduced to any family in the story, and Doug is recently single. As for the problem with Quail’s knowledge of his assassin role, he is no longer any kind of liability if he is simply kept alive in a coma and monitored. Keep in mind, he is alive after all and would still be capable of telepathic communication to the aliens if they reached out. We are led to believe that they would know whether or not he was still alive, but that is all. If kept where he is, still breathing, living out his memories as they are naturally in his mind, and unable to interact physically, then everyone seems to get what they need in order to carry on with minimal consequences.
Your idea of keeping Douglas in a suspended state of consciousness is very creative! This way it solves the problem of him being an issue for Interplan as well as avoid the invasion from coming back.