To what extent can using judaism.stackexchange in the following ways be considered fulfilling the mitzvoh of learning Torah?
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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.
While the question of the topics included in the mitsvah of Torah study, is surprisingly unaddressed in Hazal and Rishonim, we have a few sources that pertain to various subsets of the site.
Rambam writes in Perush Hamishna to Avot (1:16) as follows:
ואני אומר, כי הדיבור יתחלק לפי חיוב תורתנו לחמשה חלקים: מצווה, ואסור, ומאוס, ואהוב, ומותר...החלק הראשון, והוא המצווה, הוא קריאת התורה ולימודה והעיון בה, וזו מצות עשה מחויבת: (דברים ו ז) "ודברת בם"...
והחלק הרביעי, והוא האהוב, הוא הדיבור בשבח המעלות השכליות והמידותיות, ובגנות הפחיתויות משני המינים גם יחד, והערת הנפש לאלה בסיפורים ובשירים, ומניעתה מההן באותן הדרכים עצמן. וכן לשבח המעולים ולהללם במעלותיהם, כדי שייטב מנהגם בעיני בני אדם וילכו בדרכם, ולגנות הרעים בפחיתויותיהם, כדי שיתגנו מעשיהם וזיכרם בעיני בני אדם, ויתרחקו מהם ולא ינהגו במנהגם. ויש אשר יקרא זה החלק - רצוני לומר: לימוד המידות המעולות והרחקת המידות הפחותות - דרך ארץ.
I say that speech is divisible into five categories based on the dictates of the Torah: commanded, forbidden, detested, beloved, and permitted...The first category; the commanded, is reading of the Torah, teaching it, and delving into it, and this is a required positive mitsvah "And speak of them" (Deut. 6:7)...
The fourth category; the beloved, is speech praising intellectual and ethical perfection, and criticism of shortcomings in these two areas, and the inspiration to these [former] qualities, through stories and songs, and prevention of these [latter] qualities, with those same methods.
And also, to praise virtuous people and to praise them for their virtues, in order that their standing will be raised in people's eyes, and they will [be inspires to] imitate them. And to denigrate bad people for their faults, in order that their acts and mere mention, become detestable in people's eyes, and they distance themselves from them, and not imitate them.
And there are those who cal this category; the study of proper character traits, and avoidance of negative character traits: derekh erets; courtesy. (All translations my own).
That is, he differentiates between that which is the mitsvah of Torah study, and that which is positive, but not included in the mitsvah. The latter category includes discussion of proper character traits vs. negative traits, and discussion of great people, even if the intent is to learn appropriate middot; character traits, from their behaviour.
Accordingly, asking, thinking about, reading, and answering many questions tagged middos-character-traits, rabbis, and biography would not be included in the mitsvah of Torah study, although that does not mean that they are not valuable, and obviously questions within a tag may vary, and some may be included in the mitsvah.
Assuming that the topic of a question is included in the mitsvah of Torah study, then even thinking about the questions and their answers, would be considered Torah study according to the opinions that Torah study need not be enunciated. (Note that this seems to be the primary view).