Update Request

Go to Apple's feedback page, http://www.apple.com/feedback and click on the appropriate subject area. This current exists in iMessage, however it is the responsibility of the receiver to format their device to show Read.

http://apple.com/feedback

There is such an option for the recipient to allow you to get a read receipt (Settings/Messages - Send Read Receipts. But I think it would violate privacy rights to let you choose if messages you send would have to generate read receipts.

Of course, you have no way of knowing if someone actually read your message; all it could tell you is that they received it.

And as written earlier, the sender does have the option to provide a Read receipt to the sender for the iMessage. They have to configure it on their own device. As shown in this page from the iPhone User Guide, http://help.apple.com/iphone/9/#/iphf2d853e3 you can make a settings change to show the Read receipt for an iMessage.

It is very possible. And yes, Read receipt is off by default. The user can chose to turn it on or off at their leisure, and turning it on does not apply to older messages.

That would be up to you to do. You are the only one that can mark a thread solved or not.