Unless Zeality thought about something special, I think Belthasar just refers to the King's elderliness, to the fact that he was soon to die of old age.
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I've quickly skimmed through the plot again, and I think the Porre Agent's relevance could be increased by involving him in the Reptite timeline events.
The Porre Agent was last seen in 1,002 A.D. returning to Porre with an engine for the Chimera and a bunch of data from the future. After that, we're not supposed to see him again until the Porre Lab infiltration in the Medina Forest Ruins.
I propose that he reappears earlier, in Chapter 9 (An Emerald Dream in 1,002 A.D.). The timeline is turned into a Reptite timeline, but according to TTI, the Porre Agent should still appear in 1,002 A.D. no matter the changes (like Belthasar who appears in the future whether the Apocalypse happened or not).
Thus, during their exploration, Crono, Glenn and Magus meet the Agent. He's confused and thinks the Dispatch Chamber sent him in a wrong time/place. Some things happen (I'm not sure what) in addition to what we already have for the chapter. A fight with the Agent may happen, but he quickly realizes that they should ally themselves or something. Eventually, when the party defeats Coyopa, the Agent goes into the portal. The party jump in too, and find him in Prehistory busted by King Zeal at the Lavos crater.
The rest of the short Chapter 10 plays normally (choice between Ayla and King Zeal, etc.). Chapter 11 is also unchanged, except that when Marle returns to Chronopolis, she finds the Porre Agent also bedridden, in addition to Crono, Glenn, Magus.
Not sure what can happen after that. The Agent could play some role in Chapter 12 (Cakhula's interrogation). Or in Chapter 15 (the Guardia-Porre-Medina meeting). Or in both or somewhere else...
Ideas, thoughts everyone?