Single-ticket trip in Brandenburg, Germany

I recall that we once used Brandenburg tickets in a group of three people. Another time, some local German gentlemen helped us to buy two tickets to Frankfurt-Oder at Tegel Airport. Now, I am going to travel to Cottbus alone and want more clarity.

Which ticket(s) should I actually buy at TXL?

I guess that I can use the same root, bus+railroad. The airport recommends some VBB buses. I guess that I buy bus ticket again at TXL.

But where do you have the railroad ticket kiosks?

In VBB recommendations to travel onward from TXL, I find a trip with Bus X9 - Zoo Garden, followed by RE2. There are also some other alternatives, which are more twisted and last longer because they involve more transports and transitions. VBB says however that no matter which route I choose, the whole trip will cost 14.30 anyway. This is curious because bahn.com says that 14.30 is a cost of railroad alone whereas vbb/fares, according to their website, say that AB ticket within Berlin (or, I should take another for TXL-HBf/Zoo trip?) costs 2.70.

You see, I even don't know if I should go to Zoo as VBB guides says or to HBf in order to be sure that I can buy tickets at the rail station because nobody says where has railroad placed its ticket machines.

What should I choose in the ticket machine at TXL and which railroad stations do expose the ticket kiosks?

The same applies for the backward route: will I be able to buy a single ticket to TXL in Cottbus rail station?

On Berlin airport website, www.berlin-airport.de/de/_dokumente/reisende/2014-12-14-vbb-liniennetz-b-bb.pdf, we can see that there are many cities with ABC fare zones. But what is the point of that?

Can single 1-day AB ticket be used in both city centers?

What if I also want to use public transport in another Brandenburg city -- is one day AB ticket, purchased in Berlin, also active in AB zones of other Brandenburg cities?

How options are affected if DB re-enables its online ticket ordering?