I will try to give you some help, recognizing this is not exact science & will still require some trial & error on your part.

Sounds like somewhere in the 140-145 mm width would work, but don't be afraid to try 155. Not sure that the 160 number adds anything to the equation. You would not be able to sit on a 143 if you needed 160.

You might, or might not, need a cut-out. Just because you ride aero does not determine, it is the degree of sensitivity you have in that area. Remember that tri saddles, which are designed for riders who stay aero for long periods of time, have no cut-out, rather a wide, soft nose. Also remember that not all cut-outs are created equal & some might actually feel worse.

My assessment to your scooting back on the saddle is that the front is somehow uncomfortable & you are trying to get away from it. If you end up on the back, and that is comfortable, that width might be good. If you are sitting well back, you are sitting on the widest part of the saddle. If you are still way back & it hurts to sit up, you may very well need to go to a 155 width.

The Selle Italia you are riding is a men's saddle. If the other saddles you tried that hurt are also men's saddles, I would probably try a women's design, with a women's cut-out. They are very different.

Yes, a saddle can be too wide in the back. You will know, you will get chaffing in the back. The shape question, t vs. pear, is also a factor. The t-shape comes in more sharply, so if chaffing between your legs is a problem you would steer towards t-shape. If your sit bones are more widely spaced and your hips are broad, you might prefer the pear. My guess is if you ride a men's saddle, which is more narrow, you would be better off with the t-shape.

I can't tell much from your weight. Are you thin & slight for your height? I am 5'2" & 101, my sit bones are very narrow, so I ride a 130. I am a very active rider, so I get rubbed raw if there is too much saddle. I rode a men's saddle for years very comfortably, but have had to go to a women's cut-out model recently. You should end up with one saddle you can ride comfortably for both training & long rides. The MTB saddle I expect would be different, because you sit differently. But you should be able to find one saddle for your road bike.