Tonguez
A suffusion of yellow
Why ask if there are stories about solitary hags that curse when you acknowledge they exist in the second half of your sentence? The question is not whether the Coven also exists - it is whether there are folklore origins that are the solitary hag, and we both agree there are.And the Hags discussed in the monster manual lack key features common to their folklore origins, the Hags in the monster manual differ from the common Hag of folklore. That is the issue we're raising.
What I’m suggesting is the Hag race in the MM and the Unique Crone with curse powers might be similar but dont have to be the same creature. You can make a new monster, use a different creature or give a hag class levels that access bestow curse.
Your Goblin example kind of illustrates the point too - afterall Bugbears and Goblins dont have much similarity at all yet both are called goblins, then you expand to Orcs and Ogres in the same ‘goblin’ horde.
Also I chose Baba Yaga as to illustrate my point both because she is so well known and because she is a powerful ‘Hag’ who could be made using the Hag stat block or as a unique creature.
In some folklore She is a group of three sister all known as Baba Yaga (so the Coven fits, with RP such that only one sister appears at a time) but She was also probably more goddess than witch and so setting her up as such is also appropriate.