Best wishes for your efforts, Darren. I'm sure you will soon get the hang of what you are trying to do. (I have a friend who plays harp, and she uses chords all the time.)

Anytime I've seen a chord in parentheses, it simply means that the chord change is optional. In my experience, what the accompanist depends mostly on how fast the others are playing. If they are really ripping along, there not be time to make the change. (In that case, it may be best to let the accompaniment die out for the mo.) If the tune is going slowly, the dissonance will grate on the nerves, and the change should be made.

(Sometimes dissonance may be desired to add a note of sadness or anger, but that's just a matter of taste.)

Anytime someone wants the main note to be special, it's been indicated with a slash, like this Bm/F#. Suppose I have a measure which is almost all in Bm, but the first note is an A. By calling for an F#, which harmonizes with the note A, the arranger gets the Bm chord he wants without introducing an irrelevant dissonance.

Keep in mind that chords are not guitar chords, they are music theory. Chords can be played on many instruments - guitar, harp, piano, banjo...