Hi, Caps and welcome to the forum. I'm very sorry for your losses. It's so hard to lose several in a row.
It definitely does sound like the heater was leaking something into the water and over time, it built up and caused the deaths of your fish. Since I'm not familiar with what goes in a heater, I can't even begin to speculate what might have been leaked out but I can imagine it was something very chemical. For now, the best treatment for Lima 2 is to get her into clean water. Since she's been in aquarium salt all along, it's probably a good idea to give her system a break from it. Just use clean water and water conditioner. It would also really help her if you got API Stress Coat. This will soothe her and help her build a good slime coat.
Generally, I don't recommend anything Top Fin. It's Petsmart's own house brand and it tends to be pretty cheaply made. For heaters, I recommend Aqueon, Cascade, Fluval, Marineland. Those are all good heaters. Do not buy a Tetra heater, it is unreliable and has been known to overheat and kill. In a 2 gallon bowl, a 25w heater will be fine, but a 50w heater works too. Your temp and your pH are both okay, no need to worry about that.
As the others have suggested, tear down the tank and give it a good scrub with hot water and either vinegar or a dilute bleach solution. If you use bleach, rinse it very thoroughly until you no longer smell any bleach and then use extra dechlorinator the first few times you fill it/do water changes.
Keep an eye on Lima 2 for any signs of stress: lethargy, clamped fins, heavy breathing, struggling to swim. These are all signs something is wrong.
A good water change schedule for an unfiltered 2 gallon is 1 50% and 1 100% with a gravel vacuuming per week.
Best of luck.