Will the 170k from the old chain be burned after today?
Of course coins will be burnt after today.
But the coins that will be burnt is not the 170k from the old chain as you are asking.
It is the remainder of the premine from the new chain which will be burnt because they werent swaped for.
But then some block explorers are on wrong chain?
And what are exchanges doing to be sure that theirs wallets are on the right chain and not on fork?
They can't be 100% sure, but what are they doing to reduce the chance to minimal level?
One thing you must understand is that forks happen everyday even on the biggest blockchain including the bitcoin blockchain.
In the end the most important thing is that the longest chain is the original chain.
In fact any upgrade to any coin wallet at all is a fork on its own no matter how minute the change in the upgrade is.
What determines the longest chain is basically miners and full nodes.
When most of them are on a particular chain it becomes the original/longest chain.
The best way to avoid splitting chains is to make sure that you have correct addnodes(full node holders address) and for mining to be more decentralised.
Because like I said earlier chain splits happen everyday but it will never be a problem unless a miner or a group of miners with sufficient hashpower decide to continue mining on the splitted chain in which case it becomes a new coin that has to be given a different name.
As a rule of thumb, we say that it is expedient for every exchange or wallet or whatever that stores Solaris to avoid deposits and withdrawals a few hours before and after an upgrade.
This is to as you said reduce the chance of double spending either against you or at someone elses' expense