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I'm not sure what type of bug has infested your plant, but Neem Oil is a good treatment.
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

It needs just a whisper of water while in the dark, maybe a 2-4 Tblspn per month. Exact amounts vary with size of container, temperature and humidity. If the stems look like they are drying out, more water is needed.

I do recommend that, if you would like to save as many as possible, you will want to bring some of these indoors over the winter. They will have better success indoors, than outdoors, typically if it is under a year old. After that, there is a good chance they will survive on their own.

The pH will not give you any indication on if it needs water or not. It can only tell you if the soil is alkaline or acidic, which will help you to see what the plants may be lacking in, nutrient-wise. The moisture readers are often hard to use, and will only work if the soil is moist. The best way to check whether it needs water or not would be to put your finger down into the soil about 2 or 3 inches. If it is dry passed this point, then it will need water. If there is still moisture then I would wait a day or so. That is the most reliable way I have found to check moisture level in the soil.

Here is an article for more information on the care of the plant: https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-how-do-you-take-care-of-poinsettias.htm

An active, advanced infestation of scale insects is difficult to control with most insecticide contact sprays, horticultural oil or organic botanical repellents because the adult scales are protected with a waxy covering.

For fast and inexpensive control, I would use Orthene or the generic Acephate97Up, mixed in an ironing type spray bottle and applied directly to the foliage where the scale insects are implanted and feeding. The material is systemic and readily absorbed into the leaf surface. Its a stinky, older generation organo-phosphate insecticide, but it works, and fast. Follow product label directions for safe and effective use. It is low toxicity for mammals, people and pets, but toxic for fish and bees.
https://www.domyown.com/acephate-97up-insecticide-p-2751.html

There is an alternative that can be applied to the soil for root uptake, so you don't have to spray. Safari is also a highly mobile systemic insecticide and it is not stinky. But it is considerably more expensive.
https://www.domyown.com/safari-20sg-systemic-insecticide-with-dinotefuran-p-2512.html

After this current advanced infestation is controlled, you can take up a preventive program with twice a month spraying with neem oil or another organic/botanical type of product:

https://www.domyown.com/monterey-70-neem-oil-p-2841.html

This will not survive in your area at all. Your only hope is to regrow it indoors in a south facing window, or under horticultural lighting. It will come back, but it takes some knowledge of the plant to get it to bloom again. They will need long days of about 18 hours of light for a few months, then you will have to drop the hours of daylight to below 12 hours daily. This will induce flowering again.

Here is an article for more information on their care: https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-how-do-you-take-care-of-poinsettias.htm