Has Klingon changed significantly since it has started being used for Star Trek: Discovery? Has the vocabulary grown significantly? Has there been novel grammar revealed? To what extent is the language still developing under Marc Okrand's guidance to meet the demands of the script, and how much leeway does Robyn "Qov" Stewart have in coming up with Klingon translations for the script?
The language still has Okrand's guidance, and he trusts Robyn to do it correctly.
Before Stewart initially took the “Discovery” job, she did check in with Okrand himself to make sure he approved. “He said, ‘You know what? I’m really glad you’re doing this for two reasons: One is so I don’t have to do it.’ Because just for a movie, it’s a lot of work, and the series goes on,” Stewart said.
But Okrand then added that he knew “the language is in good hands.” Said Stewart, “It was a huge ego boost for me — his confidence that I’ll do it correctly."
And even though Robyn Steward has Okrand's approval, every change has to get his stamp of approval before officially being added to the language.
TrekMovie tells us:
As was noted in the video, Okrand isn’t working on Star Trek: Discovery, as the translating for the show is being handled by Klingon expert Robyn Stewart. However, Okrand is still the only person who can introduce new words into the Klingon language, which is maintained by the Klingon Language Institute. As he does at each annual conference of the KLI (called the qep’a’), Okrand introduced a number of new Klingon words in the summer of 2017. You can see the full list at KLI.org (words noted as “qep’a’ 24” were announced this year).
So to address each question:
Has Klingon changed significantly since it has started being used for Star Trek: Discovery?
Nope - the people working on it are hardcore Klingon speakers, doing their utmost to keep everything accurate.
Has the vocabulary grown significantly?
Yes, it's grown - you can see exactly which words were added recently in the above link.
Has there been novel grammar revealed?
As far as I am aware, and from what Google turns up, no. The grammar is still the same as it was - just words are being added.
To what extent is the language still developing under Marc Okrand's guidance to meet the demands of the script, and how much leeway does Robyn "Qov" Stewart have in coming up with Klingon translations for the script?
As mentioned in the above quote from TrekMovie, Okrand still has control over the language - while Steward has some wiggle room like any other translation to any other language, she has to work with the language, only adding words when absolutely necessary.