Wow, I could write a novel…… I have probably the most double jointed thumbs of any harpist you will encounter. I feel strongly that the most important thing is placing the thumb high on the string and closing. I always tell the student to “push towards the column” as they are playing their thumb. A person who is extremely double jointed may not be physically able to straighten their thumb. But if they can place high and close, everything else usually falls into place. I think teachers often fixate on having a “straight” thumb. A straight thumb is defintiely preferable if a student can do it, but if the student physically can’t straighten it that is not of much help. I prefer to focus on what everyone can do, which is place high and close. The combination of those two elements will usually result in a good sound and secure technique.
I should give a big caveat here that there is a big difference between someone who truly cannot straighten their thumb, and those who are just not used to doing so. You really have to examine a student’s hand and work with them for a while to figure out exactly what is going on and what their physical capabilities are. Sometimes it’s just a matter of strengthening, but sometimes it’s a physical thing that is not ever going to change.
Stressing to the student to place high and close is almost always best way to go in this situation. You may find that the thumb will naturally straighten on the string if the student can get used to placing higher.