Yes it is just Click and Drag.
I have tried it in Sierra (picture) and High Sierra with the same results.
I do tend to start at the last word and then select back to the first word I want.
Clicking in the iMessage involved tends to select the whole Bubble and therefore the "whole lot".
On the other matter of not being able to link the iPhone and Mac consistently check these things:-
On the iPhone in Settings > Messages > Send and Receive are both the Apple ID and the actual phone number both set to Receive and Send ?
The number should be ticked but greyed out ( Permanently On) but the Apple ID can be ON or Off - it just needs to be Signed in for the Sync to work.
IF the number is somehow deselected (it happens but is rare) you need to start the Message app again to get the App to login/Register the number first. Turn Messages Off in the settings and then set it to On again and wait for the Number to Verify.
You can further test the Mac and iPhone.
ON the iPhone deselect the Apple ID as a Send and Receive item (you don't need to Sign Out of the Apple ID).
On the Mac deselect anything but the Apple ID.
So at this point the Mac can only Send and Receive from the Apple ID and the iPhone only from it's number.
Start a New Message on the Mac and enter your iPhone number in full, including Country Code in the "To" Spot. (it will not call up your My Card info from Contacts).
Send the iMessage.
Does the iPhone see the iMessages ?
IF it does not then:- When the iMessages account will not Login
The last bit can be time consuming as you have to work your way up the different level of Responders to get to someone who can look at the iMessages Server and tell you if your Apple ID is blocked when used from that Mac in relation to iMessages.
They then have to get the server reset.
Apple ID issues like this are free to fix. Some people have been told they were "Suspended" because of "Spam iMessages" but this has only been reported by people posting back rarely and there seems nothing to back it up.
A "regional thing" does sound more like an action by the carrier to monitor certain ports of the internet.
In the iChat days a Video or Audio Call started on the port 5060 which is used for VoIP.
Some ISPs who were planning their own VoIP service started blocking the ports then asking for extra to use them.
Apple got round it the same way Skype did by moving away from the affected ports.
However trying to pin down which ports now the app does not do Video and Audio chats "in house" as it were would be difficult.
9:44 pm Tuesday; May 29, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)