It is possible for them to mess stuff up, there are a lot of lines in the UK, so yeah that can happen.
As for that "10 day line settling" part, that is for any xDSL lines where the connection needs to sync up to the best speeds the line can handle without errors, or something along those lines, basically xDSL speeds will be based on your type of cable and the length of it and the noise that might also affect your line, so it needs to learn those types of lines.
With FTTP there is nothing like that due to noise cannot affect it like it does with copper and aluminium lines, also it with FTTP it connects up at the maximum speed each end uses and the ISP or openreach caps that line to the profile (300:50, 500:73, 910:110 etc)
Also with FTTP it either works or it doesn't, there is no learning to be done.
Whenever I have been given that same 10 day speech I use to correct them, but most never listen even when its explained to them in layman terms, so now I just ignore that part.
I have only had a few issues with BT a while back, but since I had to get my local MP involved to get BT to enable our FTTP Hardware BT have now seem to be more reasonable when I have to deal with them.
I think it boils down to what time that you phone them, I find first thing in the morning is the best time to phone them, they are fresh and relaxed, phoning them in the afternoon and evening they might not be due to customers moaning and yelling at them.
I also have good results when speaking to BT over Twitter and its the same chatting with Openreach over Twitter as well, I have gotten several peoples information updated in my area due to errors in their database, which then allowed them to order FTTP.
Basically it boils down to how you come across to them when you speak to them, I have always been polite to them but also firm at the same time, I have always had my issues resolved in a reasonable amount of time, sure we had issues upgrading to the Full Fibre 900 package, which I had to keep on at them until it was all resolved, but I always remained polite to them
Paul