its been quite some time that i ve had experience with these guys.
1st pic surprised me, because from the top view it reminded me of Thalassophryne Amazonica, which is entirely freshwater. Last pix clears that out, because that face is not a Thalassophryne.
On the other hand, my Allenbatrachus Grunniens ( and several others I saw, were considearbly darker than yours )
It may be the Trispinosus, of which I know zilch.
Keep it as much as possible in the conditions it was in, for a spell. When increasing the salinity, do it very slowly, over a long period.
If it is eating, it is a good sign. Previous and actual owner did ( are doing ) a good job.
Grunting is a defensive, threatening mechanism that is staple with Allenbatrachus ( do not know about Trispinosus )
Mine loved shrimp. They preferred eating in the dark, so if your's is eating with light you are more apt a keeper than I
Enjoy
M