The way I see it is the first appearance of Clem in That Certain Smile was a one off, with no plans to make this a regular character. I think his second appearance in A Landlady for Smiler was the episode where Roy Clarke was 'testing' this character for further appearances in the next series.
In A Landlady for Smiler the character was slightly changed to suit the situation, but I think most changes were explained to suit continuity. There is no reason just because 'Smiler' had become Nora's lodger at the end of Series 12 that he had to remain there for Series 13.
All of that is entirely true and isn't in dispute. As I said above, in that era there were a couple of new characters introduced that "were too good to let go" in Roy's words...like Howard, Wesley, Edie etc....who were there as a one off to start with but then were re-introduced either quickly, or with a time lag, such as Wesley who waited 2 years between Car and Garter and The Loxley Lozenge, just like Clem/Smiler did.
What we are debating now, and the nub of the issue is..."Does Clem have any relevance to Smiler and are they the same character?"
And the answer is YES they are and YES he does. A Certain Smile was his intro and with a slight change in circumstances, he rappears 2 years later in A Landlady for Smiler.
We meet him in A Certain Smile where he has a Wife and a little Dog. We meet him again and what nickname has Roy chosen for him? Tinker? Laugher? Tiger? No. The little Dog and the wife ha had when we first met him are mentioned again in his 2nd appearance, but their demise has led him back into the series....He is the same character, plainly, with a touch of modification to ease his path back in.