I recommend getting some plastic screw to containers as a replacement.
Volume Plastic Sample Bottle 5ML Bottle Vial Storage Container Green Cap 10pcs | eBay
I have some like this and prefer them to the glass ones
High PH is safer than neutral or low PH for most fish. It is way harder to screw up and cause a PH crash. Even though many tropical fish come from soft acid water they are bred in hard or outright brackish dirt ponds in Asia.
If it has been planted for months I doubt your water will look bad when you get the test kit. Planted tanks can basically cycle themselves.
Unless it looks like fuzzy fungus it is probably a growth. There are a few kinds and I don't think any can be treated but most are not dangerous.
1. Does algae really kill plants?
If they stay covered in algea they will probably die.
2. what factors promote the growth of algae?
Excess light and nutrients are the main ones. Some combination of lower light levels, more water changes will greatly reduce it. Ad some kind of cleanup crew...
Unless you want to do get a microscope and do full necropsies on your fish be happy with "You did the best you could". Not every creature will live to a ripe old age even in perfect conditions and there will be outliers above and below the predicted life expectancy. For bettas the range is...
If you don't do a fish in cycle perfectly your fish are harmed by exposure to ammonia and nitrite. They might not die right away but they will not live full lives. The safer way to do a fish in cycle uses only one fish and little food so the ammonia is released in controlled fashion and you have...