Topic: Evolution of BSS version of the LoS dialect
This is only my humble opinion.
You wrote "Сhanges were so huge that next one will probably break compatibility with Shadowlandian Black Speech and mark the birth of new dialect."
If you do break with LoS I suggest that you look into the verbal conjugation-system. The feature that the verbs have a special conjugation for the third person is a bit strange and really feels like a borrowing from English. It's is not a feature of any other languages that I have any knowledge on.
I would either simplify the system so it conjugates the same for all persons (like in e.g. Scandinavian languages). My proposal is then:
infinitive:-at (as in the Ring Inscription)
imperative: -Ø (null-ending)
Past tense: -az
Present: -aX (X= any consonant)
Future: -ub
The Svartiska system is really quite like it except for the past tense:
infinitive:-at (as in the Ring Inscription)
imperative: -Ø (null-ending)
Past tense: -ul
Present: -at
Future: -ub
Or you could construct a more complex system with different personal conjugation. Zhâburi marks the difference between transitive and intransitive verbs with difference suffixes, -u- being intransitive and -a- being transitive. E.g. thrak- > thrakag "bring something"; thrakug "bring one self/come".
The transitive then marks both subject and object (not obligatory) by different suffixes, e.g. thrakaganul "I bring it". The intransitive marks both subject and object (because they are the same), e.g. thrakugul "he brings himself/he comes".