4)ご無沙汰しなくてごめん!
I believe my error was using 'shinaide'. Is shinakute correct?
8)My basic understanding of the ~tara, ~ta form I gleamed from the textbook was this that I wrote down: " 'When S1, S2'. In this construction, S2 is a happening that was unexpected or surprising."
When I originally wrote the sentence, I thought that perhaps the idea was in fact that 'I bumped into the teacher ('bumped', as yoshi also told me) and we had a long conversation' which, I thought, meant that it was a surprising meeting.
I should keep in mind that it can't be used to describe a past event in simple terms. On further thought of the construction, it does sound a little funny - I believe I was just in error about what kind of sentence it makes.
In that regard could I also use 出会って to express that 'bumped into', 'ran into' factor? If I simply use 会って, is it arguable that meeting the teacher could have been pre-planned and not a chance 'ran into' situation?
9) 'The point is what to do.'
I see... So, if the idea is missing the train, I should express that, and not what comes before it (like getting to the station). However, if I was meeting someone at the station for a date, perhaps, then it's alright to say 駅に間に合わない。
Out of sheer curiosity, is there any time to the potential form of ma ni au at all?
These explanations and discourses are amazingly helpful. Thank you so much.