Nirish, I share your frustration, but I don't bother with "but why didn't you tell me earlier..." routine...it is pointless and gains nothing for you or them, in fact may work against you. You just have to accept the system there as it is and make those emails asking for alternatives, they can't read minds of what 'might' be acceptable for you, me or anyone else. The key is actually knowing that you have to do it to get a possible room/time re-match. On saying that because I book 2-3 months ahead it is very rarely they don't have the room or dates I ask for, it only happens Jan/Feb for obvious reasons. I like the laid back attitude at Mosaik and with it comes a laid back attitude to renting out rooms.
I know they have enquiries coming in thick and fast and I suppose if they did they did try to make 'alternative' offers for every enquiry they would spend half their lives trying to accommodate, I can't think of any UK hotel nowadays who offer alternatives if the dates you have asked for are not available.
The other thing about Mosaik is that they seem perfectly happy to leave 1,2 or even more days between customers, they leave at least a whole working day between rents to allow for cleaning etc..where most places get the place ready between customers check-out and check-in on the same day, thus you are allowed to check out any time during your leaving day up to when they close reception. They just don't seem concerned to rent out every possible minute/hour/day, often this is to your advantage,even my last trip I changed plans and wanted to arrive a day or two earlier than booked...yes no problem.
Last Feb they were so booked up with short peroid stays I had only managed to get a deluxe unit which is the smallest we will take, on arrival they had re-let my room and simply upgraded us to a much larger unit for the same price.
Back to the question of alternatives, I just noted the OP said South Pattaya not essential, if Jomtien is acceptable I would have no hesitation in recommending East Suites. Their rooms are very spacious and their largest room is 100 sq/m. All rooms with large balconies and floor to ceiling sliding doors the whole width of the wall. They have 30/40/50 sqm units but the 60sqm units are a very reasonable Bt1450 per night, the 100 sqm is Bt1650, they have small kitchen area with kettle/microwave/sink/big fridge, free WiFi and are furnished to the same high standard as Mosaik, nice beds, sofas and décor. I stayed there twice this year for short stays (3 nights) and plan to again at the start of my next trip Jan/Feb.