Learn English – “When is the last time…?” OR “When was the last time…?”

Right, as you say, when we use the past continuous tense there is usually some context that makes it clear to the reader / listener when the event was happening. Your question seems to be mostly about stative verbs (1) to be; and (2) to see.

(1) Be

Verbs that are not typically used in continuous tenses are called stative. Examples are see, be, want, hear. So we say "I was in bed..." not "I was being...". There are some cases when stative verbs like "be" are used in a continuous tense and these cases tend to change the meaning of the verb.

(2) See

As to the correctness of "I saw a very big bird at 10am this morning", this will depend on the context. It is a grammatically correct sentence; it appropriately expresses the idea that at 10am a bird appeared in view.

However, if you wanted to express some kind of continuous observation then you would need a different verb, not see (which is stative). E.g. you might say, "when someone knocked on the door at 10am I was watching a big bird".

A. Working took place in the past. (A period of time is implied, of course, since you had to work for a period of time, but that fact isn't specified in this tense usage.)
B. Similar to A, but working took place over an unspecified period of time in the past.
C. Working took place over a period of time in the past. The tense specifies that there is a time also in the past that the period ended. (In your sentence, "until" is the wrong preposition; "before" is correct. Also, "before today" must imply "before your termination today"--which it does--to be correct.)
D. Working took place over an unspecified period of time in the past. My answer to C also applies here.
E. The person is still working here, and you want to know when he started. This isn't consistent with your scenario, so presumably that's why you don't feel comfortable with it. :)
F. It would be correct if the person isn't yet seeing his employment as being in the past. ("I have been working here for six years, and I'll be darned if I give up my job without a fight." would be an obviously correct usage, for example.) Once the person understands himself to be terminated, he had been working here for six years (when he was terminated).
G. Yes, I agree. "for the past six years" (or more often "for the last six years") should be used as "I have been working here for the past six years."