So Jake finally proposed?

Is her tail ok?

Canonically she doesn't have a tail. It was cut off when she was exiled from her village. https://www.furaffinity.net/view/22213593/ ScorpDK does draw her with a tail sometimes though.

Correct.

And when I draw her with a tail, it's either an ethereal one gifted to her later in the story, or an AU version of Telethia, which did not lose it. :3

So, wait, are Jake, Telethia, and Arkane polyamorous, or do we have different universes here?

He's married to both, yes.
In fact, he would be married to a lot more of his female companions, if the situations allowed for it.
eg: Princess Yue, of the Guao'pan, considers him her future husband, but he doesn't want to cause a political incident by just going ahead with her idea without proper diplomatic talks with their kingdom - which, given she snuck into his luggage to leave the island, is thinking she was abducted.