MTG land playing postcombat.?

I don't know the specific details of Two Headed Giant (i just don't play the format, so there might be some nuanced rule I am missing), but ...periods would be extremely helpful to figure out what you are saying in general.

If it is your turn to play you can play Rakdos Carnarium as long as you control a land to return to your hand. I don't know why someone says you can't unless you don't have another land. You can play a land anytime you can play a sorcery (aka nothing on the stack) with the only restriction being you can play one per turn unless you have an spell or ability that allows you to play more. That include both pre-combat main phase and post-combat main phase. You are right to be able to play you land after combat as long as you haven't hit your quote of lands played that turn.

As for your Doomgape thing: I have no idea what it has to do with your question but you can block it with 15 1/1 creatures with life link. You will lost up to 10 tokens (mattering on how your opponent chooses to split damage, but more than likely you will lose 10) due to lethal damage and you will have gained 15 life.

That you may simplest play 1 land per turn, whether or not it's a basic or non normal land. Unless you could have some thing that says in any other case like exploration which is an enchantment which says, that you can put down another land each turn.

Your friend needs to read the basic rules of the game. You can only play one land a turn, most of the time, but it doesn't matter if it's the first or second main phase.

you can only play another land in your mp2 if you did not play a land in your mp1.