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There is no schedule on how frequently kits rotate out, nor is there any grantee that a particular kit will be offered more than once, or a time after, and so on.
Nor are you meant to. Under rule 2.1.8., the transfer of an account or other game values to third persons; publication of any offers to sell, buy, or transfer an account or in-game property, both free of charge and for crystals or funds is punishable by a permenant block.
This idea will not be implemented for several logistical reasons.

In the future, if you'd like to suggest an idea, please do so here: Ideas and Suggestions.
Unity is a whole separate platform. Right now Tanki is using Flash Player as its platform. When the Unity version first comes out, there will only be a client for it, but then as time goes by, the hope is to make it accessible from a browser as well.
Simply post, "I am now leaving this clan. Please remove my name from the list."
This is because you already possess all the hulls at the same modification. The system recently implimented by the developers is designed to make it easier to get all of your hulls and turrets on the same modification level, but without having any at a higher level of modification, this will not apply to you.
The lowest rank you can play with is a Warrant Officer 3. This rank is also known as, "WO3".
This is because your version of Flash is not up to date. If you use Google Chrome, your browser will keep the in-browser Flash always up to date. If you wish to continue using the browser you are presently using, please update your version of Flash here: Adobe Flash Player - Debug Downloads.
It may be because of what country you are in, and thus your payment options are limited.
Tanki Online 2.0 is an old idea that has since been trashed. The Unity version of Tanki Online will be released in the near future, though no certain date has been set yet. You should expect to wait at least several months.