NO, this size would be 6.33% smaller in overall diameter. Never go over 3%. The perfect replacement size in a 50 series would be a 305/50-15. A 295/50-15 would be acceptable. That is assuming you have no clearance issues.
YES, no problem.
No, this is a bad swap. If you want to go with a 65 series tire then install a 275/65-15 which is almost identical in overall diameter.
235/65-15 or 275/75-15 are very good substitutes.
Not unless you change the aspect ratio. For instance: If you have a 215/70-15 tire and you replace it with a 255/70-15 this would be a horrible swap as the 255/70-15 would be 7.58% larger in overall diameter. It is never advisable to go over 3% difference in size. Besides your speedometer would read 55.4 at a true 60 mph. If you want to install a 255 tire you will need to install a 255/60-15 to replace a 215/70-15 which is an excellent swap.
It is 265 mm wide
Anything from a 245 to a 265 will fit with 15 inch rims if you have 16 inch rim go with a 255 of 31" tires
265 70 R 15
No, the difference is way too much and it would very likely rub the body while turning.
I have a 2003 wrangler x with 265-r75-15 tires and have no problems.
I doubt it and certainly not on the same rim. It's possible, but I would ask a tire shop pro to check it out.
P 265/70 R 15
Yes but the profile is different and will affect your speedometer.