Miners collect fees on each transaction upon the solving of a new block if those transactions are included in that solved block. That happens now and will continue to happen even once solving blocks stops producing a guaranteed BTC income. For the second part of that question... you can see now where some things might have to be changed once there are only fees as income. This is what I explained earlier - how the possibility of mining returning to low powered machines may happen (or not).

It would play out kind of like this...

Once the block reward becomes zero, the ASIC miners that are used now would simply be too costly to produce profit. The miners would stop using them, thus bringing the demand of solving blocks to lower powered machines. The network difficulty detects this drop in computing power and adjusts to fit the 10 minutes per block expectation as best it can. Solving blocks moves to more affordable equipment as the reason for high-powered equipment vanishes.

For your second question. I'm confused - or you are. You don't split a key or something. At one point, if a zero is added, one satoshi simply becomes 10 satoshi, as is standard math. Of course, other changes would have to take effect immediately as well such as a price adjustment or similar to keep people from suddenly having 10 times their wealth. ;)

For your last question, you're contradicting yourself. Coinbase IS an exchange. And any time I purchase there (for trading through GDAX), I do move any non trading coin off immediately. As with any exchange for any currency, they are there because they get money! Once again! K? There are fees... If you go to another country, in the airport there are exchanges. You don't get 1:1... You get the exchange rate MINUS their fee. Coinbase is in it for the fees, and of course the gains it can make itself by working with its earnings in cash and Bitcoin.