Etymology Query: "They"

[carljweber]

Etymology Query: "They"

Carl Jeffrey Weber

(I couldn't find anything by searching the archives.)
Given all that the OED says about "they" (and other pronouns) -- I have for
years carried around two questions:

1. What is the evidence behind the OED's presentation that "they" is Danish?
(a) Is it a "probably" type conclusion? (b) Is it based on similarity of
forms only?
2. Why was the older nominative all-genders plural pronoun replaced at all?
Were there developmental constraints, the result of reorganization during
the period of Norman dominance?

This "they" pronoun seems to have been the only trespasser into "basic"
English (with the other th-plurals later following). Has any native source
been suggested?