Welcome to TFF.
First issue is the GH/KH/pH. Do not use any chemical substance to lower the pH. With the KH (Alkalinity) as high as it is, this will not work long-term. The initial lowering will last a few hours, but within 12-24 hours the ph will be right back up where it normally is, and the fluctuating pH is hard on fish. The only way to lower pH is to deal with the GH and KH, reducing them, and the pH will follow suit depending upon the numbers. These three parameters are inter-connected.
The only safe way to do this is to dilute the source water with pure water. Pure water refers to Reverse Osmosis water, distilled water, or sometimes rain water. But this is a permanent issue, as every water change will mean preparing special water outside the aquarium. This can be difficult to manage, and expensive, and emergency water changes (they will happen at some point) are even more difficult when you have to prepare the water in advance.
Second, male Bettas are not community fish so he is best on his own. The cory catfish should be OK, but no upper level fish. You are lucky the neons did not go after the Betta, or the Betta after the neons, though of course this may indeed be why they died. Don't temp fate or risk fish.