Bit late, as you have had a great deal of information and help already, but here is my two-penneth worth.

Yes, there are those who are very skilled in arranging harmonies and getting exactly the effect they want instrumentally, but we all know, as you quite rightly say, Sandy, that vocally we all have our limitations. As with most things, put people into a perfect situation and you can't guarantee a perfect outcome.

So, although you say you arrange chords by instinct and know when to change because it "sounds right", it's what most of us do. All I can add to that is - experiment and have fun.

Cheers

Sylvia