I used to could stand on my head, but I can't do it now
Used to could is acceptable and grammatical in this sentence–if you are speaking or imitating certain dialects of the American South.
It is not grammatical in so-called "standard English."
Southern American English uses forms such as y'all, fixin' to, and double modals such as might should and used to could.
This may sound strange to you, but it's perfectly grammatical in Southern English.
Here's a tweet from two weeks ago:
I used to could hang with the younger generation. Now I find myself in bed before midnight.
Another example from this month:
You used to could store your archive to Google Drive (at least with the Android client).
(Source)
Then there's the song "I used to could" by Mark Knopfler (of the band Dire Straits
Live in Royal Albert Hall (YouTube)
Lyrics:
Well, all down the 40 I never used to lift
Thirteen gears, double clutch shift
All those horses underneath the hood
I don't do it no more but I used to could
GMC Cannonball going like a train
All down the 40 in the driving rain
All those horses underneath the hood
I don't do it no more but I used to could
Well I don't hang around 'cause it ain't no good
Like the big bad wolf in the neighborhood
Chasin' after Little Red Riding Hood
I don't do it no more but I used to could
I don't recommend using used to could on a test of standard English. But you need to know that there's a lot more out there in real life than "standard English."
See also Is 'might could'
a correct construct'?
As for I've never could understand him, I have to say I'm unfamiliar with that phrasing. I've never heard anyone say it.