To keep the answers short???????? and a bit limited to each point more or less I will add this note.
While in Seaboards site find Tony's articles about exhaust systems and study it and then compare to yours.
All this is in Tony's Tips.
Elbows do die. However there are things that can shorten their life appreciably with a distinct possibility of damage to the engine depending upon the systems design and mode of failure.
Read/study the article:
Marine Exhaust Systems Overview
Pay particular attention to the orientation of the riser and spray or shower heads where the exhaust is mixed with the raw water.
Then look at yours and see how it compares. You are at the point that any changes needed should be done now.
It may simply be you need a new shower head and a more frequent, much more, inspection frequency.
That missing part in the shower head could lead to a burned exhaust hose. If the water does not mix well with the exhaust gas , then the gasses are not properly cooled, allowing hot gasses to exist and maybe, burn the hose. Replace it, the shower head.
Yanmar may offer a better unit.
David Marchand wrote about his experience although a different Yanmar engine but his experience may be applicable here also.
And ask some more questions if need be. Also more photos showing how it all relates and the shower head's orientation would help.