After an arduous process of sifting through all the .MXF workarounds I could find, I decided to save myself some time and uninstall Premiere Pro then re-install it / re-activate it. Surely enough, all the formerly missing sequence settings and format support is now there. Thank you all for your insistence that I should stick to .MXF.
On another note: I opened my .MXF files and they now work wonderfully. My sequence settings are (25fps, 1920×1080, Preview file-format: 1-frame only MPEG as opposed to one of the P2 DVCPROHD options).
I shot an interview in a wide-shot so that I can have the option of zooming in to a medium shot or a close-up shot later on in the editing phase (since I have enough HD quality to go for this and since I saw this done in a number of documentary films). My question is, do I make the sequence resolution smaller than the original video resolution (1080 for example), so that I can help make the close-ups lose as little resolution as possible when I enlarge the videos? (I also want to use a lot of low-quality mobile phone videos, so I need an all-encompassing sequence preset..)