Why is Proto-Germanic long i /iː/ reconstructed as "ī"?

Question

I know that Gothic has "a large number of archaic features".
I know that Gothic writes Proto-Germanic (PGmc) "ī" /iː/ as "ei". wīną wein, swīną swein
I know that Gothic has ?non-productive? e / i alternation qens wife, qino woman
I know that Gothic has i / j alternation kuni kin kunja kins, andanahti evening andanahtja evenings
I know that Gothic has archaic ei / ij alternation eis they (plural masculine) ija they (plural neuter), þreis three (plural masculine) þrija three (plural neuter)

The first question is why PGmc long i reconstructed as "ī" (not ii)?
Could Old Slavic long i И be I + I? Cyrillic alphabet often enough uses almost horizontal lines in diphtongs.

Answer 1

The first question is why PGmc long i reconstructed as "ī" (not ii)?

From the comments, it sounds like you're asking why it's written as *ī. And the answer is really, historical accident. There are a few different conventions for writing long vowels, including macrons (ī), acutes (í), IPA length marks (iː), and doubled vowels (ii). And various people involved in the reconstruction of Proto-Germanic chose to use macrons, so that's now standard. (That's also standard when talking about Proto-Indo-European, for related reasons.)

As for Cyrillic И, it's not related to I; it comes from the Greek letter Η ("eta"). For a long while the Greek alphabet wasn't standardized, and some varieties used that letter for an "h" consonant sound, while others had lost that sound and repurposed the letter for a vowel instead. The Latin alphabet descends from the former, the Cyrillic alphabet from the latter.

(Other ways of writing /iː/ include stretching I into ꟾ, or using a completely separate letter. It all comes down to history.)

Answer 2

The symbol macron goes back to classical antiquity, and was used to notate a long or heavy syllable. That diacritic was therefore widely used in historical reconstructions, and is used in the vast majority of the defining works on reconstruction of Indo-European and Finno-Ugric. It is also standardly used for Arabic, Japanese (historically and otherwise), and in the 19th and early 20th century for Native American languages. Double-vowel traditions compete: use of a colon is a more modern practice that has not generally overturned the traditional means of writing vowel length. However, there are traditions where long vowels are written as double vowels, such as in proto-Bantu. The explanation in the case of Bantu is that macron clashes with circumflex, and there is a vowel quality difference written as i/î in earlier works.