Who installed that flue? Is it an existing install that's been modified?
The gray section is for a roof mounted flue not an exterior elevated flue.
There are very strict guidelines around flues and flue elevations.
I've not listened to your video but I will do shortly just putting the kids to bed.
Ok so listened now...
The plumber is correct you cannot have a bend in the pipe as the exit flue and intake pipe (flue is actually a intake and output pipe) have to be pointing the same direction. It's less to do with carbon monoxide and more to do with the vapour dispersing in a way that is correct, (to do with wind on the flue). The bend pipe you are talking about is called a plume kit and is only designed to be used on a wall exit not a chimney exit as you have (although th chimney exit you have is questionable). The right angle in th flue as it comes out the wall is called a diversion joint and you are limited to the number of them you can have in a flue.
The lower pipe that MUST be accessable is called a blowoff valve, it's the pressure release point if the boiler were to fail, the blowoff would drain the boiler of its contents, and have the potential to be at boiling temperatures, all new blowoffs have to be crimped to form a fan dispersal paturn so the water cools quickly.
As for the Engineer comment, your plumber is probably not gas certified, therefore is not a gas service engineer. Gas service engineers have to spend an absolute fortune getting qualifications and years of experience before being able to work solo! They then have to repeat the qualifications ever couple of years at great expense. Just for reference, I have a few different gas qualifications and I spend the best part of £6k every couple.of years to remain qualified.