a) If you thumb through the Union Hymnal, they selected pieces which were largely adapted from tehillim. The idea was to lend the synagogue some dignity and decorum and some class. Chief Rabbi Hertz, zt”l, cites Isaac Watts’ hymn “O God Our Help in Ages Past” (psalm 90) as “a cherished spiritual possession of the English-speaking race” on page 887 of his Humash and on page 150 of his “Book of Jewish Thoughts,” he includes the hymn verbatim. Likewise, the Hampstead Synagogue, which was one of the best representatives of Minhag Anglia, sang Handel’s Hallelujah Chorus, upon the 100th anniversary of the shul’s consecration. Rav Yisrael Moshe Hazzan (Kerakh Shel Romi 4b) even encouraged cantors to sit in churches in order to listen to their music and writes there that when Jewish aesthetics are beneath those of the non-Jews, Jews are duty bound to adopt the higher standard of aesthetic. Singing religious songs in the vernacular therefore wouldn’t (And shouldn’t) be considered a violation of lo telechu, when that issur is understood as a prohibition against adopting pagan practices from non-Jewish religions. Singing religious hymns in the vernacular doesn’t strike me as such, especially if you believe Christianity to not be avodah zarah. Hasidim borrowed many tunes from Poles and Slavic peasants. The Syrian nusach hatefillah is largely borrowed from Arab musical traditions, with the system of maqamot that is used in Muslim worship. Ashkenazic nusach hatefillah bears many similarities to Gregorian chant. We furthermore acknolwedge the existence of non-Jewish prophets and “Ma Tovu” is part of the Torah, despite its origins.
b) The rigidity of what we think of as Orthodoxy, and the very relegation of halakhic observance to one denomination, is a function of circumstance. Most historians agree with my observation; prior to Reform, our sages were flexible in a way that Orthodox Judaism today would never entertain. The Hatam Sofer’s polemics, as well as the polemics of many others, were a visceral, knee-jerk reaction against the excesses of the reformers. Do you seriously think an approach which looks at critical manuscripts and acknowledges them in a way with bearing on halakha (as did the Gra) would be allowed in today’s Orthodox Judaism? Many things which are halakhically allowed we don’t do purely because we’re afraid of looking like the Conservative movement.