As far as you can. If you have a friend or spouse who is willing to do it, they can push it farther.
No more than 2.5 cm
Yes you can, I have done it. Put KY on your middle finger and the pill on top of the finger and push as far as it can go, I have 5 pills, the small ones are little harder to insert. But ok, the pills worked slower....
This depends on how far up your rectum the tampon is. If you can reach in and pull it out, then do that. To make it a bit easier, you could try lubricating your rectum (kitchen oil should work fine, but body or massage oil might be preferable) - just pour the lubricant into your rectum, and hopefully you should be able to pull it out without much trouble. However, if the tampon is so far up your rectum that you cannot easily pull it out, go to your local emergency room and have them remove it for you. Either way, you're likely to have a sore rectum for a few days.
farthest
You will know. Everyone is different. 6-8 inches depending on your size should be enough. The colon/intestine make a turn where it changes from the large intestine to the small. While you can make this turn with practice, it is not recommended and should be considered risky.
By deref
3 inches
This is actually where they start, pressure from consipation, labour or hard bowel movements force them to the outside of your anus.
A protruded anus could be indicative of a hernia. A vet should be consulted to properly diagnose and correct any issue.
check the instructions listed in those fleet enemas
2/3 of the shoulder-to-umbilical distance
http://www.greatlakesgelatin.com sells it, they say you can get it at health food stores, and if you don't need very much gelatin Knox Gelatin is available at supermarkets.