The operations for cart cannot be provided in minicart as it would require AJAX to solve the issue.
We are not using AJAX anywhere in the project except checkout.

Looking on it.

Sure, thy will be done.

This functionality is required according to customer perspective because it is not mandatory that user will checkout with his default address , what if user want to checkout with other address,in this case he need to go to his profile first and then change his default address.
You can take the reference from below attachment:
https://prnt.sc/lljir6
Also you can take the reference from amazon

This is required because no one want to fill address everytime for checkout. In magento also if user have checkout with any address then at next time of checkout that address will appear on checkout page, if user has use multiple address than all those address will appear on checkout page.
Please check the magento checkout functionality for this.Also please add an option on checkout page to add new address at time of checkout.

It ain't possible to do so because it requires translations for certain fields, so the users will find very difficult to enter translated values in a single form.

Orders are not dependent on channels, even the whole admin panel is a place where all the channels can be managed in a single place.

Only forms with AJAX capabilities can have such ability but we ain't using AJAX in our project.
So the possibility of implementing this is very narrow and almost ignorable as an enhancement also.

This will be done in the next release.
Will post the commit hash here when this gets done.

Design team issued all the design guidelines and they have been properly followed.

Design guidelines have been followed.

We are looking into the possibility of doing so in the next two upcoming releases.

This will surely be in the next release.

This will be fixed in the next release.