After such a short period of developing carbonation, the fizz you are finding may be mostly what was already fermented into the cider.
Its a double-edged sword ... youve refrigerated because you dont want bottle-bombs, but this also could also prevent the full carbonation of the cider. Something of course that is to your own tastes in any regard.
It is common for the plastic bottle to firm up quite a bit, fairly rapidly. Its when they really get good and hard that you are feeling the awaited results of this test.
The real test is by periodically opening a bottle to check the carbonation.
You've mentioned you like its carbonation now ... but on the chance you want to more fully carbonate it, and while I'm waiting for some guests who are (very) late arriving and I'm sitting here bored to tears ... here's a bit about further carbonating.
You did over prime the cider, but possibly not quite to the degree that it might first seem ... in other words not anything like double or triple etc the amount you should have ... maybe more like only by another half as much as you should have.
I agree ... these bottles of cider are possibly destined to be an explosive batch if left to carbonate fully without stopping the action at some time. But, if ... IF you want, I think you can salvage it as a fully carbonated batch of sparkling cider.
The following is approximate ...
your can of apple juice, when reconstituted, should yield 48oz of 1.040 SG liquid. While a gallon of 1.040 liquid contains approx 14oz of sugar, 48oz then contains 5.25 oz of sugar.
You also added an additional ½ cup or 4 oz of additional sugar. (nevermind it had molasses in it)
5.25oz plus 4oz = 9.25 oz sugar total.
½ oz sugar added to a gallon = approx 1 volume of additional CO2.
9.25 oz priming sugar to a gallon is 18.5 volumes ... or when distributed over 5 gallons = approx 3.7 volumes.
Add to that 3.7 volumes of CO2 any residual sugars in your cider IF it had not yet fermented totally dry, and also the volume of CO2 that had installed itself into the must during fermentation, and I think you are actually somewheres approximately in the low to mid 4s in terms of potential volumes of CO2 if allowed to fully carbonate.
This is not entirely unheard of for some brewing and even vinting. Some wheatbeers (notably Dunkelweizin), some Lambics and Belgian Sour ales have CO2 up to about 4.5 volumes (this would be the very maximum). Some sparkling meads get above 4 too. Cider is not uncommon at 3.5 to nearly 4. The question of course is what will explode one of your bottles.
So, I would say that if you want to unpack all that cider from your refrigerator and watchfully let it carbonate in a proper temperature of say 70* to 75* and under a blanket ... opening a bottle every couple (or so) days, you can still get it to carbonate properly, and then either pasteurize or put it back into the icebox. I would think this could make a very pleasant, just slightly semi-sweet, sparkling cider.
Certainly if you like it as is, and don't want to risk having to re-paint the ceiling etc, you might want to keep it as is. On the other hand, if it were me ... I like a lot of carbonation.