There are a number of steps involved in setting up an email account in Kmail. Besides addint IMAP and SMTP account you have to make an identity for it

1. Setup IMAP account
2. Setup SMTP account
3. Setup Identity for it.
4. Make sure it is using draft and sent folder from the same account in identity settings.
5. Make sure it is using trash folder from the same account in the IMAP settings.

I don't understand how all accounts using a default identity from some account is a good idea. How is using identity of account 1 with account 2 is useful?

Most people are accessing email on a number of devices specially mobile. If I move a email to a trash or sent or draft which defaults to local account then that email in pretty much disconnected from rest of the world.

Gmail is a very contentious issue. If a message has several labels then in Kmail I will have to mark it read in all folders.

The issue is in GMail's implementation of its labels in IMAP, not the other way round. In any case, there was a GMail specific resource made for KMail, but it was disabled because it had too many critical bugs.

On the other issue, I'm not sure the PIM developers are aware of it. You may want to write to the kde-pim at kde org maling list asking for more input on why it is this way before filing any bug report.