Odd beer missed target gravity -- prime or not?

Odd scenario:
I brewed a crazy gluten free beer with tons of spices and no hops -- fermentables were 6# sorghum, 2# dark candi sugar, and 1# lactose. (have mercy on my recipe -- it's experimental for a celiac friend)

OG was 1058. Pitched Cooper's Ale yeast in starter. Got off to a good start, fermented in primary 3 weeks, racked to secondary based on time and not hydrometer (I recognize my error here). Let sit another 3 weeks and prepared to bottle. Took a reading, and got 1038. Really high. I've done sorghum before and it fermented out just fine (1010ish range). Let sit another two weeks, thinking it just needed more time. No change at all.

I like the way it tastes, even sweet as is, so I want to bottle. But how??

Can I rely on the sugars still present to carbonate? Or do I still add priming sugar? I've never had a bottle bomb and I don't want to start now...

It's a weird situation I've never found myself in before...any advice?

You could try increasing the temp, see if it picks up; if not, pitch in a bit more yeast - champagne yeast is great for making sure things are finished. If nothing happens even after that, you'd be safe to prime and bottle. Either fermentable sugars are gone, gravity notwithstanding, or something in there is preventing fermentation. Either way, no bottle bombs (though in the latter case, no bubbles either).
Don't bottle now; if you can rely on the sugars present to carbonate, you can rely on them to carbonate far too much. You only need a scant few gravity points to carbonate, and a scant few past that to start busting bottles.