1. How is the case prep different than the usual prep for MBB/Tier1s?
If they say you will have a case interview, 90% should be the same. As in, you still need to train the exact same skills.
Here's some critical reading for you: https://www.preplounge.com/en/articles/pitfalls-case-interview-preparation
2. How deeply should I prepare about the company strategy?
What do you mean?
I think you mean how well should you understand there? You absolutely need to do your research on the company. You need to know what they do because you need to convince them you know why you want to work for them!
Two identical candidates sit before you. Will you pick the one who did their research and knows the company (and is eager to join) or the one who didn't?
Here's some reading to help you here: https://www.preplounge.com/en/tell-me-about-yourself-interview-question
3. Since I already come with a Consulting background but was working mostly with external clients, will the work be any different after I enter the firm?
Yes, if you are doing internal consulting there will be differences in the work. That said, there will be a lot of overlap. This is a great interview question! (at the end. Make sure you word it properly)
4. Will working in an inhouse consulting restrict my options to proceed into MBB/Strategy&/Kearney after 1-2 years? The industry is Energy & Utilities in Germany and I want to move to the Middle East after a couple of years.
Not at all! Getting exposure/experience generally improves your candidacy. Of course, this doesn't help you get to the Middle East at all. Make sure to get a lot of projects, progress rapidly in the company, build your network, etc.
5. There is absolutely no selling involved inside the firm. I'm worried about not being exposed to any BD. Will this affect my chances of recruitment with any other firms in the future?
Each role will have pros/cons. Not having selling should impact your chances of being hired as a consultant, but it does affect you the higher up you go (project lead, principal, etc.)