I am not seeing a real problem. Sure focus is not sharp but you are really challenging the AF system that in the D3300 has few pixels in its metering sensor which is used in AF function. The test shots are all underexposed enough that getting as close as you are getting is surprising. The shot of the little boy is strongly backlit and a deep shadow picked as a focusing point.. AF performance decreases as you move the FP further from the center. It is possible to have a decentered lens element where it is ok in the center but more out of focus above or below the focus point than in the opposite direction of FP.
Slanted targets are not very good for isolating an actual focusing problem. A proper test would include a target in perfect perpendicular plane to the axis of the lens, with strong lighting, and high contrast. Between each test shot, move the focus ring far off focus so the servo has to focus, overshoot and return enough times to home in on the optimum point. If you do not defocus between shots, a threshold of "close enough" might not be exceeded so the software does not even start the servo.
Use only the center FP. That one FP is the most sensitive and is cross polarized so AF will be best in any light, while the outer FP's should only be effective when the contrast edges are in the same polarization as the FPs and properly exposed. The FP needs to be well exposed, so all of these images were going to be iffy as to whether they would be properly resolved. The D750 has a lot more cross polarized FPs so will perform better on the outer focus points on most subjects. It is also more sensitive, and can focus deeper into shadow, a bias of 3-4 stops....that is a lot.
Color shift focus point also, so your testing needs to be in good light that does not have a color cast.
It is interesting that the D750 does well with a 35mm 1.8 because of the 3 1.8s, it is the only one that is DX image circle, unless you are allowing to automatically switch to crop mode. In crop mode the array of focus points covers far more of the image circle, but will have far less resolution than the finer pixel pitch of the D3300, it should be lower resolution output than the D3300 in crop mode.
The distance from the flat chart should be about 50 times the focal length of the lens tested, flat on a wall and well lit without too much clutter. A single horizontal dark line in a white background is enough and in fact a better test than the many focus testing charts floating around the internet.
That is going to be the more accurate test but if you want the 45 degree angled test, make sure it is very well lit, and very close to 45 degrees from the center axis of the lens, with rigid tripod, remote shutter release. It is a rare lens that is far enough off to result in missed focus on such as large object as a face from 50 times the FL of the lens, missing focus is more often error in use of the camera....poor contrast of the AF target, poor light, color cast of the light or AF point target. If is ever gets it right, it is surely no the lens. If even 80% or off, it is unlikely to be the lens.
When looking at images that are out of focus class them as out of focus, in focus or slightly displaced focus. The out of focus shots are almost always a operator error, but the ones missed by a degree, look at the focal point, and notice just what the single pixel row sensor saw. What there a clear contrast edge crossing the FP sensor line? If not, like in the case of the boy's deep shadow eye, it is not an indicator of a lens problem. The af system had ambiguous information to work with. If it got close, it is just because of the miracle of modern software made some effective guesses.
Another thing to consider, in complex fine detailed targets is that the FP box in the VF is NOT where the actual single row AF sensor is located. It is generally close or shifted to one side of the VF displayed box. The sensor that is not cross polarized is a single column or row of pixels, so narrow you would not be able to see it without a microscope. Cross polarized FP have two rows crossing each other forming a 90 degree angle. The center FP has that so the contrast edge does not have to cross the a single row. Due to the outer FP sensor rows single lines, you will see that flipping the camera on a tripod 90 degrees yields different results. One of the orientations will be much more accurate focusing on a focusing target that has a single contrast
edge.
So, based on the above, there is no clear indication of a problem with the AF or lens, too many elements supplying ambiguous information to the AF system to do what you think it should be doing. Do ANY shots with the D3300 and 35mm lens appear in focus? If so, concentrate on giving the system a better chance of delivering what you seek...pick optimum focus targets with highly defined contrast edges, that are well lit, and do not have a strong color cast. If they are all off, you likely have a problem with calibration and a trip to the service center might be in order. Both the lens and camera should be sent in since the problem is the interaction of the two.
Note that with such narrow and displaced rows or columns of pixels in the AF sensor, if you have to zoom in on screen very far to see the out of focus condition, you might be just seeing the displacement of the sensor row from where it appears to be on the rear screen. It might be perfectly focused on the point that actually falls under the sensor, but is showing the FP box in the VF centered on what you intended. If zooming far in is required to see the out of focus condition this is likely the cause of the dissatisfaction with the capture. When shooting a scene,take that in consideration, just how much depth of field will be enough? Distance from the object and distance of the object to background, that ratio is the usual deciding point. Shooting wide open requires all conditions to be optimum for a successful capture with good balance between needed DOF and subject isolation. Shooting wide open is asking for lower keeper rate so take the same shot several times just to make sure, by defocusing between shots and picking a variety of focus points on the same plane.
Good luck