Slide lock release looks fine. I don't see how that could go in upside down.
Not upside down, but reversed (front/back).
Extractor tension seems reasonable.
Since Glock doesn't offer their extractor bar gauges to US armorers, you don't really have any way to determine the "reasonableness" of the specs and extractor "tension" by eye or touch. Maybe some excessive gouging might indicate a spec issue, which could induce "tightness", but
some rubbing and scratching is considered normal.
I ended up having to try 5-6 extractors (and a couple of RSA's and ejectors) before my new G26 was suddenly "just fine", and operated as well as my older G26. This was while working closely (calls) with Glock repair techs over some time (as it was a supplemental G26 for me, and I wasn't in a rush to trouble-shoot it, having other things going on at the time).
But what I do notice is that the slide does not slide on or off smoothly and at an even angle. It seems as if the part of the trigger bar assembly that sticks up is off center. Half way through sliding the slide on you've got to angle it a few degrees to line up the slot at the back of the slide with that art of the trigger as assembly that lines up. And when removing the slide, you feel the friction and have to move the slide off center to complete removal. I don't ever remember that with my gen 3. Seems very unnatural
Some Glocks may "feel" different in such respects, for different amounts of time, but each gun would have to be considered on an individual basis before determining whether it was "unnatural". Some can just be a little stiffer in such ways, especially when new (and especially sometimes if we're comparing smaller models to larger ones).
It sort of sounds like you're just trying to talk yourself into getting rid of it. Your prerogative, of course. Doesn't mean the G26 can't be put into perfectly normal functioning condition, and wouldn't serve you as intended, once whatever is causing this issue can be identified and corrected.
The last troublesome G26 I had come across my bench was due to the owner installing some expensive aftermarket parts, and what appeared to be a factory RSA spec issue, all of which were easily resolved by replacing the aftermarket parts with new OEM parts (including a 3rd RSA).
The last obviously problematic RSA I remember was in someone's NIB G27, where early during the very first range session, the front of the RSA came apart and the spring came out the front of the slide, causing the pistol to seize up. A new replacement RSA put the G27 back in normal functioning condition.
Sometimes in an occasional parts-is-parts guns (drop-in parts that don't require fitting), it may take trying one or more parts in any particular gun. I've been told that in armorer classes for a few different makes/models of good quality guns that see LE/Gov use.