Yes to every question you asked.
As long as the Companion Pass holder has a "paid" ticket they can add a companion to the flight. "Paid" includes reservations made with points, gift cards, vouchers, travel credits, and credit card. You will need to pay the taxes for the companion's fare and it can be paid with almost all the previously mentioned payment methods (no points and not 100% sure about vouchers).
The companion is always a separate reservation/transaction so don't worry about that when considering mixing of payment methods for other travelers. You cannot mix points with any other payment method so as long as you are mixing gift card, voucher, travel credit, credit card you can book all those in one reservation with a maximum 3 different payment methods. I would suggest booking one-way trips to break up the costs and reduce the number of payment methods you will need to use on the reservation.
Finally, yes you can edit your posts. Click the gear icon in the upper right corner of your post and select edit.
Hope I got all the questions
--TheMiddleSeat
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