Earning XPs only - i.e. without earning any corresponding miles {as would be the case when you take a flight which earns both miles and XPs} - is not a mileage-extending activity. So, making donations of miles to charity to obtain XPs is not a mileage extending activity, because you are not earning any miles. It has absolutely no effect on your mileage expiration (and merely decreases your mileage balance).

(And the miles have to be earned; buying Flying Blue miles is not a mileage-extending activity. Whether and if making a further transfer from your credit card into Flying Blue miles would count as a partial extending activity that would extend your mileage expiration, I do not know, but I would not count on it).

As a frequent flyer programme, it's set up with flying in mind; Flying Blue has been focused on the customers of Air France and KLM more than being dreamed up as a loyalty programme to reward US credit card holders; you're therefore operating on the fringes of the programme, where you're not doing the "normal" thing of refreshing your mileage lifetime continuously via flying, which is how the programme is intended to be used by the overwhelming majority of members. Miles earned from flying will definitely extend your total mileage balance by a further 2 years. Flying Blue is not a programme designed to cater for those who who refuse point blank to fly at all, let alone once every 2 years.

If you can't/won't take a flight, then my advice would be to make a booking with the miles before they expire - and this time, stick to your conviction and actually take the flight(s) you book. Remember that "changes" in Flying Blue involve the cancellation of the existing booking and the creation of a new booking - so you have to cancel even if you just want to make a small change. When you cancel a ticket, no miles will be refunded to your account if they would have expired in the meantime. So, making speculative bookings also does not count as a mileage-extending activity.

Alternatively, if you hold status in on of the airlines listed (which includes Air Canada, Alaska, American, Jet Blue, WestJet and United, among others), have you consider paying for a Flying Blue status match? If you secure status prior to the end of the month, your miles should be safe for the duration of your status.