khaki pants are never formal. you can wear them to a semi formal but black or grey would be more appropriate. the outfit you described would best be used for an afternoon dinner or a casual wedding 20 or fewer people... go to a nicer clothing store and ask a sales person. you may need to shell out a few dollars but it is better than looking dumb you might meet the person you can spend the rest of your life with. but they might be slightly shallow.
A blazer is a semi-formal, casual jacket.
It is still considered formal to send an invitation to someone who should be invited but cannot attend.
Jeans are never worn for formal occasions.
The groom's father is expected to wear formal clothing to the wedding.
This is generally still considered to be tacky, although it depends on where you are sending the invitation. It also depends on how formal the wedding itself will be.
Only if it's a formal wedding.
Yes, if it's a formal wedding - it is a way to respect the bride and groom. But if the wedding venue is based on beach areas you can consider beach wedding dresses that might better to show your appearance.
Generally, the accepted language for a wedding invitation is formal. If the wedding is going to be less formal, then the invitations should follow that example.
"Those attending Kim's wedding must wear formal clothing."
Tuxedos can be very expensive to buy. It comes down to the question: Do you frequently attend formal affairs where they are required or preferred? If so, buy one, and make it a good one that will take a lot of wear. If you are going to a wedding and have no need for a tux in the forseeable future after the wedding, then rent.
Depends on how formal the wedding is and the color of the suit you are wearing.
If it is an evening formal wedding, then yes.