Is a late addition of sugar into wine a month after primary ended detrimental?

I'm thinking your fermentation is probably just crippling along now.. and adding more sugar may just make them stressed out..and potentially cause off flavors.

But really... I think this really depends on what your goal is here. If you goal is to increase the ABV.. then you can consider adding more sugar. If it did not take off.. and you want more ABV, then you'll have to make a yeast starter and get those yeastie beasties healthy again.

If you goal is to sweeten

Then you will want the fermentation to die out & finish naturally. Then you'll ant to stabilize with potassium sorbate & campden tablets. Given it a day or two.. then backsweeten.

I would rack the apple wine off of the yeast cake and mix in your sugar and some campden tabs. I wouldn't add it back to the yeast cake. There is plenty of yeast in suspension that will be able to ferment the sugar just fine (that is, until the alcohol levels get too high).

When the alcohol levels do get close to too high the yeast will be stressed so the less yeast cake with loads of dormant/dead yeast to be cannibalized etc., the better.

I'd siphon the wine, add the sugar, then carefully add back to the cake to avoid stirring up the trub as much as possible. I'd certainly let the yeast do their thing for another couple of weeks to avoid over pressuring unless you want a sparkling apple wine, and in that case, I'd bottle in champagne or heavy glass bottles. The ABV is going to be pretty stiff and I'd worry about overpowering the apple flavenoids. You certainly will want to let the wine age a while (at least a year or so) to allow the flavor to develop over the alcohol. I had a similar experience with a pear cider that turned into a pear wine, took about a year and a half for the fruit flavor to recover. Apple is a more robust fruit flavor than pear, but I'd still be concerned.

Keep in mind that the yeast will eventually shut down as the alcohol concentration gets too high for them. So, you may not see any further CO₂ generation (seems to be your goal), but I'd really give it some time before bottling.