OK, I have successfully imported every message I wanted to import.

What I had to do to achieve this was not all that pretty Messages from particular contacts refused to open in Conversations and I had to remove the name (most of my messages had the contact name in the CSV file instead of their numbers) and replace them with their phone numbers in +XXXXXXXXXXXX format.

Also as the last 1-2 characters in all messages were omitted, To avoid this problem I added a full-stop, or an exclamation mark at the end of each message in the CSV file(s). I had stripped the number of messages to transfer down to a little more than a hundred so it wasn't all that hard a work for me, but for people with more messages, this would really take a long time to do.

I deleted el.db once again (and for the last time I hope), rebooted the phone, ran the commands in xterm, rebooted again just to be on the safe side and when I opened up Conversations, I had all the messages I needed, and in good shape...

As I've said, my solutions wouldn't be suitable for people with a large number of messages to import. But for smaller numbers, it's certainly doable...