> Is this just another name for a ledger with auditable history?
Yes.
> Can a blockchain exist without mining or without a currency/coin?
Yes, but the interesting ones use currency/coins because you can use incentives other than the legal system to secure the database.
> What keeps people interested in a blockchain (and currency) once all of the easy mining is done eg, for people joining later?
Securing databases without using the legal system in order to outmaneuver some issues intrinsic to legal systems.
> - I see that transaction costs for bitcoin keep rising, is that related to the size of the blockchain?
It is related to many things. In bitcoin's case, the primary cost is the consensus algorithm, ie the network has to "outpay" any potential attacker.
> I thought bitcoin was supposed to be decentralised – doesn’t a group of centralised developers have the ability to change everything about the currency?
This is a nuanced topic, we could go in depth about "what does 'decentralized really mean", or even worse, "what does 'bitcoin' really mean" but I find this too hard to think about.
Yes.
> Can a blockchain exist without mining or without a currency/coin?
Yes, but the interesting ones use currency/coins because you can use incentives other than the legal system to secure the database.
> What keeps people interested in a blockchain (and currency) once all of the easy mining is done eg, for people joining later?
Securing databases without using the legal system in order to outmaneuver some issues intrinsic to legal systems.
> - I see that transaction costs for bitcoin keep rising, is that related to the size of the blockchain?
It is related to many things. In bitcoin's case, the primary cost is the consensus algorithm, ie the network has to "outpay" any potential attacker.
> I thought bitcoin was supposed to be decentralised – doesn’t a group of centralised developers have the ability to change everything about the currency?
This is a nuanced topic, we could go in depth about "what does 'decentralized really mean", or even worse, "what does 'bitcoin' really mean" but I find this too hard to think about.