Here is a summary of various entities mentioned by C. Saket in the above question:
No citta vithi run in nirodha samapatti. Life in the body is maintained with kammic energy. No vedana, sanna, etc. Maximum time in nirodha samapatti is 7 days.
Parinibbana state is just like nirodha samapatti. The only difference is that there is no “coming back”.
Samapatti basically mean manodvara citta vithi will flow continuously. There is no falling to bhavanga or to take an external object with a pancadvara citta vithi. Thus one cannot see, hear, etc. Normally, samapatti will eventually break on its own or (when one gets good at it) one can pre-set the time to be in samapatti (in one’s mind).
Arahant phala samapatti is where an Arahant experiences the pabhassara citta, a pure citta with just the universal cetasika. Nibbana is the object made contact with phassa cetasika and vedana are sanna are based on that (we have no idea about that). There, one does not hear, see, anything either, just like in jhana samapatti.
In jhana samapatti, jhanic citta flow continuously at the monodvara, and there is no opportunity for pancadvara citta vithi to arise. That is why one does not hear, see, etc in jhana samapatti. A different mechanism.
In jhanas (not jhana samapatti), jhanic (manodvara) citta vithi are intermingled with pancadvara citta vithi. Thus one can hear, see, etc in jhanas.
So, I think answers to questions (1)-(4) are given above.
(5). Gandhabba is not to be taken as an entity that “exists”. As with everything, the Buddha rejected “something exists” AND “something does not exist”. Something is in existence at a given moment due to a cause. If the cause is not there, the “thing” or the “entity” is not there. So, during nirodha samapatti, the causes for a gandhabba to exist are not there. So, we cannot talk about a gandhabba during nirodha samapatti, during which time the life is maintained by kammic energy.
Another somewhat similar case applies to a being in the asanna realm. There is no gandhabba there, no citta. Just a fine body is maintained by the kammic energy. When that kammic energy expires, citta start flowing and a new bhava will be grasped.
Also, it is important to remember what the Buddha said: “there is no movement of the vinnanakkhandha without the other four khandhas”. We have discussed the sutta references in another discussion or a post; if someone remembers, please give the reference to the post or the disussion. So, there can be rupakkhandha without the other four, like in norodha samaptti or in the asanna realm.
(6). One has not removed avijja when in the asanna plane, and thus will first come back to the human realm and is likely even to go to apayas, unless attaining Sotapanna stage while in the human realm.
One gets to nirodha samapatti by taking Nibbana as the object. Thus one had removed avijja completely.
(7). The only similarity is that one would not see, hear, etc while in either Ariya or anariya jhana samapatti. The differences are due to none of anusaya being removed in anariya jhanas. For example, kama raga anusaya is removed at the first Ariya jhana, and avijja anusaya is removed at the fourth Ariya jhana.