Is Ní ach dlíodóirí iad not possible?
No. The ní in Ní dlíodóirí iad is the negative form of the copula. For "am/are/is only", you need the negative form of bí - níl ach.
I don't understand the use of Nil in front of the sentence. You are stating what they are not what they are not.
It's the common Irish structure for saying "only" (or in this case "just"). Níl ach cúpla focal Gaeilge agam. "I only have a few words of Irish.", i.e. "I only speak a little Irish." Níl ach trí euro i mo phóca. = "I only have 3 euros in my pocket."
If you speak Southern American English, it'd be something like 'They aren't but lawyers.' It's just a better generap English translation that's given.
In other English dialects, think of it as “They are nothing but lawyers” — you’re still stating what they are, despite the negating “nothing”.
English dialect construction 'nobbut' ie naught but, helps me to understand this