How can I simulate the process of marking a text (SMS or iMessage) as unread on my iPhone?
There's technically no way to mark a message as unread, but there's sort of a hacky workaround way to simulate it.

Hold down on the message you want to mark as unread and click "copy". Then start a new message, address it to yourself, paste in the message and send.

Since there's a slight delay, you can back out of the conversation, and you will have 1 unread message from yourself.