I imagine the A359LRs will have a different J product compared to the regular A359s (with a difference between 'regional' and longer haul A359s), as well as a generous Y/Y+ seat, again like the A345s (initially) had. Nice upgrade for whom it happens!
Ah, although those A345 seats were sold as economy, they were not discounted which make them more expensive than the J class of other airlines.
It won't be as necessary with the A359ULRs though. Pilots can easily keep their type rating current with the necessary t/o and landings by flying the standard A350s (I doubt SQ will have a pilot group dedicated to just the ULRs). With the A345s for most of their time at SQ they were the only A340/A330 planes in SQ's fleet. Weren't the flights to CGK less common in later years?
On reflection, you are right on that point of the coming A359ULRs being a non-orphan fleet as opposed to the A345s, so likely that them being used for JKT will not be like that of the earlier mixed Y+ and J configurated A345s.
Yes, the later refurbishment which resulted in the birds being all J class subsequently see them flying less often to JKT. Don't know whether those all J seats were being sold as J or non-discounted Y, my guess being the former.
They might still fly the new ULR fleet to JKT on the same basis as the later configured A345s; ie likely selling the seats as all Js as configured. 100 or so J is not that many to fill in that very busy sector - there are plenty of wealthy individuals and high government officials shuttling between the two points frequently. I personally know of a few Indo businessmen who fly to SIN on Friday evenings and head back to JKT on Monday mornings virtually every week...and they are on J all the time.