This sentence is unclear. In Polish pronounciation, the Ę at the end of "Kupuję" is not voiced nasally. Therefore, it is not possible to ascertain whether it is first person "I" or third person "he/she". In this case, a pronoun such as "JA kupuję" or "On/ona kupuje" should be added to the sound file in order to be clear.
Perhaps you are right. I would still assume the sentence was in first person, as the use of third person without a pronoun might require some additional context.
I agree - also in many of the pronounced phrases the nasal vowel is difficult to hear - Perhaps that's how it is in spoken Polish but for our learning purposes the speaker should make an attempt to enunciate the vowels.
The assumption that ogonek (that hook sign) is marking the nasals is a simplification. The exact pronunciation "rules" are pretty complex and it may differ noticeably from region to region. Sometimes there is no actual nasal vowel there, and that doesn't only happen at the word end.
The simplified pronunciation rules would say that you should attempt to make that nasal sound and if you won't get exact, it's still okay.
'I am buying a comfortable armchair' meaning, I'm doing it right now, I'll be doing it sometime in the future or both?
Pretty much both. Of course it has the same restrictions on when in the future it can be as the English Present Continuous tense.
It is the accusative form. For male inanimate nouns it is the same as the nominative form.
But it is. It's just that "fotel" is inanimate, so the whole noun phrase "wygodny fotel" is inanimate, and therefore it is identical to the Nominative one.