When "recycling" shares, must all cash in the plan be used, or is it okay to use just the contribution amount, and leave any existing cash in the plan alone? Is consistency required, or could all cash be used at one time, and only the cash from the contribution another time?

My advise would be to follow the document and be consistent.

You should also follow the cash flow in the plan to see if there was enough cash at the time of the recycling (including any transactions to that date) to fund for all of the recycling.