205/50-15 or 165/60-15 are excellent substitutes. Just make sure you do not have clearance issues with the 50 series tire.
Yes, that is an acceptable swap.
You can, 55 = the width of the tread
Yes, you can substitute the 205/75-15 for the 215/70-15.
Yes they could .
Yes, that is a good swap.
No, that is a very bad substitute. If you want to go with a 70 series tire go with a 195/70-15 which is a great substitute.
185/15
15 inch tires from 185 to 225 width.
205/60-15 or 175/70-15
You can but your speedometer will be off. Want to go wider then go with a 195/60-15.
Yes, that's a reasonable change.
185 /65/15