Singing in a choir is certainly a very rewarding challenge at whatever level of difficulty and perfection one chooses. Mendelssohn was more at ease with instruments than with voices when it came to artistic expression, but his craftsmanship was flawless in all genres.

Neither do I find a grammatical fault in Mr. Rutter's lyrics (whereas in the original text by Luther, the treatment of double negation is no longer accepted by pedants nowadays). You seem to think of the meaning

Thou art alone our sword and shield, Lord, which do guide the nations.

This would be quite a questionable statement, in my opinion. A sword can help us fight, a shield can protect us (when held by an experienced fighter), but these two tasks are not sufficient to guide the nations, not even in military dictatorships. May the Lord do it Himself.

To speed up learning vocal parts, I have recently lost all reluctance to resort to software. I am using "VanBasco's Karaoke Player" and/or "VocisMagis", which both work with MIDI files.