Morning,
I would write via email - today, Pentax (where ever the international warranty cites) - explaining that you had contacted them and were advised to go through the retailer (wasting valuable time), and that your warranty is expiring - today. That, should tie your warranty claim to a date within the warranty period, just in case that becomes an issue.
The overall problem that you are experiencing is not that it has been damaged and when (within the warranty period), but that it was built incorrectly, in that you have lost 20% of the lens's focal range - 55mm which is significant. I would also think that would help.
The other thing is to just persistent - Pentax should make arrangements to accept back the lens and to repair it. Sending you back to the retailer was in my opinion - just flat out wrong.
I would also follow up through the chat line again, on Monday - again to tie the claim to a date - and cut and past in to a file I would save (and perhaps print it out). I have read a couple of other threads over time, that Pentax is usually not that date specific (to the day, hour, second) on their warranty period.
Hope that helps, and hope you are having a very Merry Christmas....