2) A 235/75/15 is 29". A 205/75/15 is around 27" Yes, they will fit with no modifications. They "might" rub the frame a little on full lock, but this can be corrected by adjusting the steering stops, or just leave it because it doesn't really hurt anything for the amout you have it full lock.
Here's a quick tire reading explaination: 235 is the tire section width in millimeters. 75 is the aspect ratio for the sidewall height, it is a percentage of the section width. (75% of 235mm = sidewall height). 15 is the rim diameter in inches. To figure the height of a tire, you take the sidewall height, multiply by two, convert the millimeters to inches then add that number to the rim size.
3)Rim widths are typically 5.5", 6", 7", 8", 10" and even 12"+ These are not mentioned in the tire size number, but should be within reason to the tire section width.
4)If you're not driving off road, mount an agressive 235/75/15 on the stock rims and that should look masculine enough. A stock Samurai rim looks better than the Geo rims, and will also stick out .5" further on each side, also eliminating that rubbing on the frame

5) 5x5.5" or 139mm