I see others have addressed your in-game bug issue with the Legion Starter quest so I will address the overall issue of in-game help from GMs.
The answer mostly is no, they don’t assist with in-game issues on a per-account basis and the GMs/CS team don’t take Bug reports. They never did, with rare exceptions I will get to in a minute 
QA takes bug reports which can be filed via the in-game bug report tool, or the Bug Report forum. Once filed, it is up to QA to document, attempt to replicate, and work with Devs to prioritize. Devs fix bugs in patches that fix it for everyone at one time.
Fixing for individual accounts tends to make things worse once the actual bug source is addressed, so they don’t give the GMs tools to do it, and don’t let them by policy - for the most part. There are rare exceptions where GM are given the instructions to modify individual accounts for a specific issue approved by the Devs. They also gave a lot of the GM tools to players so they can resolve things themselves (item restoration, char undelete, char stuck, guild leader dethrone, mail restoration, etc.)
GMs also are not allowed to give Game Hints, which is what a lot of the quest help comes down to. They will send a template telling you to check guides on Wowhead, ask your fellow players, etc. If that still does not work AND it is something they are allowed to assist with, you can re-open the ticket and request further help. Sometimes they can assist.
CS has never been allowed to give game hints, has never been the venue for submitting bugs. The extra tickets that go in for it just further delay help for things they do assist with. Technically we don’t pay for CS at all - and the CS team supports ALL Blizzard games and services, not just WoW. Our Subs have not gone up in 20 years, and we don’t get anything beyond access to the game servers for those subs. Any other things like Forums, Bnet chat/launcher, Customer Support, etc are considered “perks”. I am sure Blizzard feels CS is a very important “perk” though which is why they offer it.
You can read more about what GMs can and can’t do here