I would have to disagree with the claim that 10-15 mins a day is not beneficial. You need a strong meditation practice before undertaking a retreat, especially one as long as 10 days. For most people on this board who don't meditate, sending them jumping into to an intense retreat would just be cruel.
From my experience with meditation, I believe it is more beneficial to sit regularly for short periods of time than it is to sit for a really long time once a year. Of course ideally you would be sitting decent lengths of time everyday and taking occasional retreats.
I am very glad though that you mention how you mention the power and influence of perspective and perception when you write:
"these changes are not catastrophic or damaging in any way, unless you decide they are."
I feel that I'm similar to you, in that I've been on both sides. I have done time in a very dark HPPD hellhole and today I'm living with HPPD taking a seat in the background. I think one of the turning points for me was realizing that these changes are permanent and becoming accepting of that and the "new me." By not demonizing every single visual that I experience I was able to begin to live free from the much suffering caused by HPPD (or really, caused by ourselves in reaction against HPPD).
In the Vipassana tradition it is common to do a 10 day silent retreat. I'm not sure where you can find one that is free because they cover your accommodation and food and everything.