Yes
yes
Yes.
it depends if you add leap years
Any planet with people who use calendars would need to invent leap years.
No ! The presidential elections are not always on a leap year. Even though a leap year is every 4 years, not every election is a leap year. The notable exceptions are years that mark centuries (example 1800, 1900, 2000). If those years are not divisible by 400, then the year is not a leap year.
About every 7¼ years, all the seasons would be starting a month later according to the calendar.
No, only planets inhabited by beings that need calendars have leap years.
Because of the extra weekday each year, and because of leap years, identical non-leap year calendars repeat on a cycle of 6 or 11 years. Leap years repeat every 28 years. (There are only 14 different possible calendars.) The years that were the same calendar as 2012 were 1984, 1956, and 1928.
The 2000 calendar will repeat in 2028. This was a leap year and leap year calendars generally repeat every 28 years.
6 (3 regular years, 3 leap years)
Yes, it was a Leap Year in Australia in 1996, the same as for all other countries using the Gregorian or Julian calendars.
The Julian calendar has more leap years. Every 400-year period of the Julian calendar is three days longer than the same period in the Gregorian calendar.