I would definitely return a lens having been used so little and having any problem. I have done business with Adorama, and although I've had no equipment issues, they have a good reputation and seem like they would be very cooperative.
I have had very good luck with Pentax. I originally started with a metal-body Vivitar SLR and 50mm lens in the mid-1970's when there was no auto anything. 6 years later I bought my first Pentax, the M Super, and was amazed at its aperture priority automation. I stuck with Pentax for my first AF body about 7 years later, and ever since. I've had just 2 issues over these many years and a lot of equipment. I still have several 35mm film bodies- PZ-1p, MZ-S, ZX-L, and ZX-5n, and Super Program.
My first DSLR, the *ist-D began draining batteries. It was still under warranty, and back when Pentax had their own USA repair facility, I got it back rather quickly in perfect working order. I went on to also get a K-100D and then the K-200D and K-20D. I especially loved these two for their fine build, fine IQ, and WR. I still have them all. The K-200D developed the battery drain problem, even though it has had low use. I should get it repaired before selling it, or sell it as is.
I have also owned a K-r, and still own a K-5, a K-5 IIs. I recently acquired a KP. I have a ton of glass by Pentax as well s 3rd party models. including top-quality DA*, FA*, and Limiteds. Your experience comes as a surprise to me.
My approach to telephoto is to use the highest-quality APS-C body, especially one providing the best higher ISO/ low-noise performance, and with high-quality fast or fairly fast lenses. For the same framing as a FF body, the APS-C allows use of a smaller, lighter telephoto lens. I have the DA* 50-135mm f/2.8, the DA* 200mm f/2.8, and FA* 300mm f/4.5 as well as the DA HD 55-300mm f4-5.8, and older Sigmas: 70-300mm f/4-5.6 APO, 150-500mm f/4.5-6.3 which tested at f/5.6 by Pop Photo. I haven't used the last two for years. As to FF (the K1), I look forward to getting one for use more in the wide angle to short tele range. My K-5 IIs has been very good at ISO up to 3200. My KP knocks at the door of the K-1 for low noise and overall IQ, which it was designed to do, and I am knocking on wood that I continue having good luck with its having no service issues!
Last edited by mikesbike; 08-27-2017 at 02:18 PM.