It'll diffentiate into root if deprived of light and has enough moisture... but the moisture is a risk for buried stem, too. I don't think it's a high risk, but if it's wet constantly there's a chance of infection as stem is not supposed to be buried/wet constantly. Eventualyl it will differentiate into 'root', though.
I've read to stick to 1-2". I've never had that problem. I'm sure you 'can' get away with more, but probably starts to elevate risks at some point -- more stem surface buried, greater chance of infection?
More importantly, have you fixed the cause? Have you increased light? The top few internodes look like a better length, but still may need more light to avoid continued stretching. Internode length is a guide to adjusting light intensity / maximizing light.
I wouldn't bury down to the cotys. I'd maybe bury a couple inches to put the cotys at a more normal height. Consider LST. Bending it over will help conrol the height and if you provide the proper light after that, height should not be a problem.
To be safe, stick to a good wet-dry cycle for your substrate for 1-2 weeks after burying it. Don't allow wilt, but don't keep it wet 24/7, either. If soil, let that top 1" dry before re-irrigating. Can push it later on because whatever you bury will eventually differentiate into root structure.
To see that in action - check out those "cloning balls" they sell. you attach a little bulb of moist substrate at a growth node and within a week or two you have roots growing mid-air (inside the bulb of course but not underground). A plant is very compartmentalized as far as how it behaves. Bury a portion and it'll grow roots. You can train a branch down, bury a portion and it too will eventually sprout roots. It simply reacts to its local environment.