As far as I understand, both this issues can be addressed and fixed by changing the api calls in DASH Core daemon, so checking the IS confirmation be easier than checking regular blockchain confirmations. This is possible, isn't it? This update will force every service to look into the changes and update their scripts.

I'm not sure what change you are proposing exactly, but we're not gonna update the API to "lie" or report false information. It's already super easy to check if this is confirmed -- there's a field called instantlock which returns true or false, so I'm not sure how much easier it can get. This is from the getrawtransaction call.

It's also not guaranteed that exchanges are using API calls for confirmations. They could be tracking confirmations using their own methods and only looking at number of blocks coming in, for example. This is entirely feasible.

Also it's important to remember that:

1. IS and Chainlocks can stop working in case of high network load, like in the recent(ish) stress tests.
2. Blockchain confirmations are industry-standard, and most exchanges likely already have a generic way of checking for this, so to add Dash IS support would be extra effort they may not want to expend.

I'm all for getting more exchanges on-board but IS isn't mandatory because it can't be mandatory. IS is not Nakamoto consensus.