It is not appropriate to do Nitnem while the rainfall of Gurbani is going on in Ran Subaaee Kirtan. What is there in this world, that is more worthy than Akhand Kirtan Akhaada? When Akhand Kirtan all night is going on, at that time one should be actively engaged in looting the true profit of Naam and Gurbani Kirtan. Nitnem can be done later but the opportunity to earn Laaha of Ran Subaaee cannot be earned later.
Having said that, doing Nitnem during Ran Subaaee Kirtan is not a Kurehit though; just not wise. If Nitnem is done during Ran Subaaee Kirtan, then the Laaha of Nitnem will be earned but simultaneously Laaha of Kirtan will be lost but if Nitnem is done after Ran Subaaee Kirtan , then one would still get Laaha of Nitnem but without losing Laaha of Kirtan. So what's the wise thing to do? Do Nitnem after Ran Subaaee Kirtan and earn Laaha of both Kirtan and Nitnem.
The whole purpose of doing Ran Subaaee Kirtan is to sit through the Kirtan and earn true Laaha. Except for Langar Sewavaars, no one should be doing anything else during this most auspicious time. Nitnem is done after the Bhog Ardaas of Ran Subaaee. This is the Maryada Ranglay Sajjan - Bhai Sahib Randhir Singh jee and his companions - followed.
As for eating food after Ran Subaaee Kirtan, one should not eat Langar unless one has done one's morning Nitnem but Degh is an exception and can be consumed right after Ran Subaaee Kirtan. This again is based on life of Ranglay Sajjan - Bhai Sahib Randhir Singh jee and his companions.
More thoughts on this topic:
n normal circumstances it is true that Nitnem should be done after Gurmat Ishnaan but in circumstances where Ishnaan can't be done immediately for whatever reasons, Nitnem can't be held off for that.
Singhs have been in jails where they could not do Ishnaan for many days but they did not hold off doing Nitnem Paaths. Many times, after Ran Subaaee Kirtan, Gursikhs have to travel back to their homes without doing Ishnaan. In such situations, it's not wrong to do Nitnem first. In many cases, it may take many hours to reach home and do Ishnaan but Nitnem should not be held off till that time. Nitnem should be done immediately at first opportunity after Ran Subaaee Kirtan.
Since Gursikhs stay awake during Ran Subaaee Kirtan and attentively listen to Gurbani and Naam, they are exempt from Amritvela Ishnaan which is required to purify the body and mind before Nitnem. Ishnaan still has to be done but is done after Ran Subaaee Kirtan, at first opportunity and Nitnem can't be held off till one gets opportunity to do Ishnaan.
To sum it up - both Gurmat Ishnaan and Nitnem should be done at first opportunity after Ran Subaaee Kirtan and the one should not be held for the other.
Guru Sahib knows better.