CPL should only be used when you need to reduce/cutoff reflection from non metallic surface I.e glass It should be taken off when you do not need this feature as it reduces light transmission and will induce reflection if the CPL is not coated.
Average flash tube generally have a lifespan of about 3-5k shots, after which the flash power will slowly deterioate but it will not stop flashing completely. If budget allows, go for the 580mkII
The diode that trigger the internal flash is spoilted. My guess is that the trigger voltage of the nissin flash may be the cause of this. Anyway you have to bring to canon service centre for a more accurate assessment.
The lens is definitely spoilt. Encountered the same issue previously and get a quote of $100 from canon to have it fixed. My advise is to get a new lens instead of sending it to repair.
My personal opinion on wide angle converter is that it is definitely a cheaper alternative to achieve extra wide angle. The picture qualify of course could not be compared with the dedicated version from the makes of original but some converters out there will still produces reasonably good...