A- It's to do with how many posts you have made. there's some sort of scale in the software.
B- It depends on the context. I would use 'in' if I meant that they had a similar job to mine. If referring to the office, etc, where I work, I'd use 'at'. I wouldn't use 'on', but maybe it's differnet in American English.
C- There are certain exceptions for 'yet'- I have yet to do it = I haven't done it yet. You can use 'still' with a positive or a negative- it just emphasise4s.
D- There are two types of separable verbs- ones where it's optional, like your example, or where you must. Sadly, you can't tell without checking what kind they are.
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