1. Has anyone interviewed as a non-native German speaker at B2/low C1 level? I'm still working to improve- the cases in particular would be difficult for me in German at this point but I'm learning.
In my experience some offices have a "fluent / native German" requirement, in which case they may not even invite you to the interviews once they check your CV (always be honest with language skills on your CV, otherwise this may lead to rather unpleasant situations). If they do invite you, it is very likely that they'll test your German skills a bit, but will run most parts of the interviews in English.
2. Is there a phone interview? I read a lot about phone interviews sometimes, then other times I only see info about the Potential Test/PST
This may have changed, but the phone interviews rather used to serve the role of a "practice interview" where you do a case with a consultant to prepare for your real interview. If there is a real telephone interview upfront, it will probably involve a simple case and a lot of questions on motivation and background.
3. What language is the Potential Test/PST in? English or German? What languges are the other portions of the interview in?
See 1. PST is likely to be in English, interviews for German-speaking candidates in both English and German and if German is not required in English.
4. What was the turn-around time between Test / 1st Round / Final Round? I know it varies but I'd just like an idea.
Strongly depends on time of the year and candidate pipeline. I would guess 4-5 weeks between online application and 1st round, up to 3 weeks between 1st and final round.
5. If I'd like to start January 2019, should I apply in September? October?
Latest in October, typically 4-5 months before the desired start date is a good time
6. How much time is generally given between receiving an offer and the start date?
Strongly depends on how fast you make it through the application process and how many candidates are already booked on the initial training. Typically you indicate your earliest possible start date when sending your application and discuss it once you made it to an offer. I have seen people who started only a month from their final round as they were already done with university, did not have to move etc. At the same time you have people who receive their offer and go on a 6-month world trip afterwards...
Hope this helps!