To what extent can using judaism.stackexchange in the following ways be considered fulfilling the mitzvoh of learning Torah?
Technically, yes. Everything you describe is an aspect of learning Torah. Even just reciting the Shema or a few psukim or one liners from Tanach or Talmud (both agadata and halacha) count for fulfilling the Mitzvah.
However, just as we don't use the phrases said after the birchat Ha'Torah in the morning to absolve us from studying the rest of the day, so too, you should not let your yetzer hara fool you into thinking that using the website is good enough substitute for setting aside a time to study "properly".
And you most definitely should not justify the site into causing you to "steal from your boss" by saying that you are doing a mitzvah such as Talmud Torah when you should be doing work.