My understanding is that FJ, though in a coma, is in good hands because DH is an extraordinarily skilled medicine maker. He did, after all, manage to create a medicine strong enough to erase her memories AND the only person able to notice was the Phoenix, whose age rivals DH (and phoenix seems to be the best medicine man).
In terms of powers, I think that this is a more grey area. A lot of novels by different authors have a similar theme of characters 'depleting x amount years of cultivation' this is exactly how it sounds. A great example of this is in Journey of Flower, when Bones has a LOT of her power depleted at various times, and then Bai Zi Hua interferes, and this is when they start coughing up blood a lot.
However, individuals like DH who are older than time and born with innate gifts, I don't think it's a problem because it also seems like some people are just more gifted and DH is one of them. And he is so old that it probably doesn't really matter at this point he probs has reserves (sounds weird) . This may help: https://immortalmountain.wordpress.com/glossary/wuxia-xianxia-xuanhuan-terms/#cultivation
I always thought that because the novel is primarily from FJ's perspective we don't really see how terrifying a person DH is (because he is pretty terrifying) based on the random tidbits of others. The only ones who seem to not really care are the fox tribe, and that Chiwa guy from the demon clan, who in spite of his beauty, is universally considered not the brightest bulb. Sorry if my answer isn't helpful :/