So now Alice and Charlie can both send ~10 mBTC to each other by none of them can receive. In order to do that they'll have to increase their receiving capacity. Can you continue this sentence further?
No... unless there is another route from Alice to Charlie (or vice versa), there is no way they can send to each other...
Alice can send up to 10 mBTC to Bob.... Charlie can send up to 10 mBTC to Bob... but currently, Bob has no capacity at all, so is unable to send anything to anything to anyone.
So, the Alice->Bob and Charlie->Bob routes are essentially "one way" at this point in time... as Bob->Alice has 0 mBTC capacity... and Bob->Charlie has 0 mBTC capacity.
Alice 10mBTC ----- 0 mBTC Bob 0 mBTC ----- 10 mBTC Charlie
Now, if Alice was to send 5 mBTC to Bob... then it would look like this:
Alice 5 mBTC ----- 5 mBTC Bob 0 mBTC ----- 10 mBTC Charlie.
Now... Charlie could send up to 5 mBTC to Alice... as that is the max capacity available on the Bob->Alice route. Alice is still unable to send anything to Charlie, as the Bob->Charlie route still has 0 mBTC capacity.