Yes, that near left one looks bad on inspection. I hadn't even spotted that. (This was my very first join, so I made all kinds of mistakes). I'm wondering if I can pull up just enough track - say, an inch either side of the joiners - to open up enough space to remove the old joiners and insert the new.
Yep, I learned about pre-joining after I struggled so much with the first one and then watched a bunch of videos I should have watched before. The idea about staggering the joins is new to me and makes a lot of sense. I will put it in my back pocket for next time. I'm intrigued about the suggestion to have the sliding and fixed rails opposite each other - I see the value - because everything else I read/watched before now said that the sliding section should always go to the outside.
Well, it's definitely personal but it's not offensive, it's just true

Again, this was my very first joint, and my experience with soldering was minimal too. And not only were (to some extent, still are) my skills poor, so was my soldering iron, which I have since replaced. I have also watched a bunch of videos since then and had a lot more practice, so at this point I would say that my soldering is close to adequate.