I'd skip on the hammock and get some type of floating plant instead, the price should be about the same and they will like the feel of a real plant better. Though i've also seen bad reviews on 'betta hammocks', so i may be basing it on that a tad, it's up to your judgement x) If you do get real plants for them tough, be sure to wash them off under the tap with hot water first so you avoid anything they may be carrying getting into your tank.
For the heater, I'm not sure it really matters, but i'd keep it in the middle of the tank so the temperature will remain mostly the same on both sides. It shouldn't burn them or anything, and if it does, get a new one- it's defective and dangerous. Sorry to hear you can't get anymore fish, but at least your bettas will be happy ^^