>> The manual says that it's AI-s, but nothing on the lens indicates it.
There is nothing on the lens that says "pre-AI", "AI", or "AI-s". The difference is in the metering coupling. For more details, read
this article.
>> if the lens can be mounted without one [an adapter], there is no loss of light/clarity.
That is correct. Among Nikon lenses, pre-AI lenses are the easiest to mount on Pentax DSLR bodies. AI or AI-s lenses can be mounted but not very solid (the coupling ridge gets in the way). You have to be extra careful - the lens may fall off.
Do not attempt to mount Nikon lenses onto Pentax
film camera bodies. You may damage the body, or the lens, or both.
If you can mount a Nikon lens onto a Pentax DSLR body, you can focus to infinity - no problem.
I played with Nikon lenses on Pentax DSLR bodies for a while, and collected quite a few Nikon lenses and film camera bodies. I however got tired after a while and sold most of them (still have 2 F2 bodies, an FE body, an 85mm f/2 and a 50mm F/1.8 listing on FleaBay any day now).
Note that there is no coupling between the Nikon lens and the Pentax body, so you have to stop down the lens yourself. Also, Nikon G lenses (without aperture ring) will always be at smallest aperture.
>> Did you say that I can turn on K7 without a lens attached it would take pictures?
He wrote "be able to shoot without any lens attached." He didn't say that it would take pictures. Yes, the shutter will trigger without a lens, but I wouldn't call what is recorded on the sensor a picture.