Using rough estimates, your store bought apple juice may have been around 1.045 and adding 1 C (0.5 Lb) of dextrose would give another 4-5 points of gravity. So you likely made a 5% ABV cider, the quality of which you will have to assess. Hopefully you measured the original gravity. Store bought (highly processed) apple juice can sometimes make decent cider, other times turns out harsh. It depends if the juice had the necessary nutrients the yeast need to ferment cleanly. It often helps helps to add nitrogen (YAN) and other nutrients. If the fermentation is happy/healthy, you may not need to age as long. Most ciders benefit from some level of aging - I have found that a month is generally the minimum and that also helps to clarify the final product.
If you backsweeten (if you feel it needs it) the fermentation will re-start due to the presence of yeast. If you backsweeten, then you need to add potassium metabisulfite and potassium sorbate to kill yeast activity. This is especially important if you backsweeten and bottle at the same time to prevent bottles bursting. Apple juice is a fine option for backsweetening and will add apple flavor and aroma. Some people backsweeten with non-fermentable sugar (such as splenda) and then you can skip the sulfite, sorbate additions.
Refractometer correction factor should be similar enough between wort and must without needing to worry about it. I have noticed a fractional difference, but question my measurement accuracy.
Oxygen impacts cider and it's generally best to avoid exposure after fermentation for shelf stability. For a small 1 gal batch that will be consumed quickly, it may not be a major concern.
I can't provide any advice on methanol risk. It can be a trace byproduct of fermentation in small amounts but I have not worried about it if the ingredients (wort, must) is high quality and the yeast was healthy. Hoping others can weigh in.