Fern,
I went to KL in Mar this yr via Jetstar. Since it is a short flight, it really make not much of difference if u take SQ or budget airline. U can opt to upgrade ur seat for jetstar to the first row to enjoy more leg space for $20. Cos for infant, there will be no seat allocated. u got to sit him on ur lap during take off and landing and the crew will give u an infant safety belt to strap ur bb with ur own safety belt. the flight is very short and fast. so if u can keep him busy for 30 mins or so with toys or bb biscuits, it will be fine.
as for hotels, since u need to prepare food for Elias, mayb u can see if there are those rooms that come with electric cooker. Javier is taking milk and outside food when we were in KL. I bought the Pigeon instant food too but he don't really like it. (he wants to eat what we are having on the table when we were outside. I pack FM in tupperware container and check in. In additional, I took 2 milk feeds on board in milk dispenser, took thermal flask filled with hot water. the crew allowed me to carry them on board. and took his tiny bolster for him to hug during the flight in case he is afraid or bring his favourite toy. I din get a playpen in the hotel as Javier sleeps in between us..actually he cant sleep well cos he is not familiar with the place...so we had terrible nights....need to bring him down to hotel lobby to coax him to sleep then carry him back to room. I stay in Royal Bintang hotel. quite a nice hotel. they had 1 at Damasara and 1 at KL.