You are weighing the likelihood of missing the flight against the certainty of paying for two tickets versus using Avios and a companion voucher that will otherwise become useless.

Unless the cost of the paid flights is enough to affect your lifestyle in a noticeable way, which I suspect it isn't, the downside of possibly missing the last flight of the day, being unable to get another flight on short notice, and totally messing up a trip of this size would tip the scales for me.

Besides, maybe between now and November you'll be able to figure out something else to do with the voucher!