The browning is actually enzymatic browning because of the breakdown of the banana cells. There is no such thing as too brown, but out of experience I know that brown and soft bananas could get mouldy.
The mechanism which turns the banana starches into sugars is actually the same as that in brewing beer. Amylases (amylase enzymes), in this case alpha-amylase and a bit of beta-amylase, turn the starches into different kinds of sugars. Alpha-amylases turn starches into unfermentable and fermentable sugars, with more unfermentable. The beta-amylase turns unfermentable and complex fermentable sugars into simple fermentable sugars. Boiling denatures (deactivates) these enzymes.
Freezing will not denature the enzymes. Experience from keeping bananas in the fridge has shown me that they get brown even faster. But that could be because the ethylene the bananas create, through which they ripen, gets trapped in the fridge.
You do not boil the bananas to extract sugars, although you could get the same results as with brewing. At a temperature of 70°-72° C, the alpha-amylases are very active and will turn the starches faster into sugars. However, the enzymes will have lower (or no) mobility in a banana than in a mash. This means that they will reach less starches to convert.
Since the sugars in the banana are formed by alpha-amylase activity, you will have much unfermentable sugars. However, alpha-amylases also turn complex, unfermentable sugars into simple fermentable sugars, but at a much lower rate than beta-amylases (which are much less present in a banana).
If you want to control the ripening of your bananas by measuring OG, I suggest that you take a sample of 10g banana and add 90g water, mix well and then measure the gravity (with a hydrometer or a refractometer). This will only show the total amount of sugars, it will say nothing about the ratio of fermentable sugards versus unfermentable sugars.
Sanitation of fruits for making wine is done with sodium- or potassium-metabisulfite.