Thanks, Lang, for posting the relevant links!
The short answer to the question, ” is there a limit like nirodha samapatti on how long an Arahant can stay in Arahant-Phala samapatti?”:
– No. There is no limit to how long one can stay in Arahant-Phala samapatti.
– That also holds for jhana samapatti. In a regular jhana, one goes in and out of jhana (and could be aware of the surroundings.) But in a jhana samapatti one is totally absorbed in the jhana, and only jhana citta flow uninterrupted.