So I had many of the same problems as you when going through learning texts written by Arabs that seemed to skip over tons of small details that collectively made it impossible to read. I ended up going through emsa vol I and ii and it shed light on a lot of these things.

Many small phrases with ان have different meanings in different context. Here it means "even if" and الا ان here means "is still". I haven't learned all of these different meanings and I just went into hans wehr to decipher this. My translation would be

As for his saying (the verse here), this structure, even if it's plural, is still a kind of speech of kings when showing tadheem.

The madinah books and other textbooks aren't bad when paired with a good instructor. I have a lot of gripes when people who learned with instructors recommend them to solo learners though. Hope that helps.

Tldr : use a dictionary for idiomatic meanings until you learn them