If your ear and tuner give the same results, then just to it by ear. People's pitch differentiation skills vary. I was a volunteer in someone's university project where they were testing if classically trained musicians had better pitch differentiation than other people (a silly premise but there is often an ulterior motive to these tests that you aren't told about). My pitch differentiation (I was played 2 tones and asked whether the 2nd tone was lower, higher or the same as the first) turned out to be better than 3 cents (which is a fairly common electronic tuner accuracy) but a lot of people do have much lower pitch differentiation
The truss rod shouldn't be hard to turn unless it's either at the end of its travel or there's some swarf caught in the threads.
When you say that the neck is 'still slightly curved towards the strings in the middle of the scale', can you just confirm that you are saying that the neck has a slight upward/convex bow to it, rather than the slight downward /concave bow in the middle that you would expect?
If so, the truss rod needs to be loosened (rotated anti-clockwise). From loose, tightening it first reduces concave bow, then makes the neck flat and then introduces a convex bow. See the diagram below which should hopefully remove any terminology misunderstandings.
So if your neck is slightly convex, you need to loosen off the truss rod. Depending on how well levelled the frets are, you can get away with anything from flat to quite concave (never convex), but the best overall action without buzzing is normally found with a very slight concave bow in the neck.
Loosening off the truss rod nut won't be risky, even if it does feel a bit stiff. Sometimes a squirt of WD40 or a drop of penetrating oil down the truss rod hole will help make things move easier (WD40 is less likely to leave a stain, thpigh the oil may be more effective). If still stiff after several turns, it's probably worth removing the truss rod nut completely, cleaning inside it and the end of the truss rod threads, adding a drop of oil and hopefully it will be a lot smoother when you put it back on.
If you currently have a concave bow, and the rod won't go any tighter when turned clockwise, then it may have reached the end of the screw thread - or there may be swarf or dirt in there. So it's still worth undoing the nut, then cleaning the nut and the thread, putting a drop of oil in and see where that gets you.
If that still doesn't work, loosen the truss rod right off, turn the neck upside down, support the neck at both ends than hang some weights from the middle of the neck for a day or so to try and encourage the neck to flatten. This can work, so it is worth trying.
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