It's your money but
a) yes you can see it in the sun as much as any GPS - that shot above was at 2 pm with a very bright sky....no LCD screen can be seen if the sun is directly on it and you don't angle it away - I leave my X-mount slightly loose on the bar so I can tilt it..
Generally iPhones have brighter and bigger screens than GPS do and certainly more detailed ( more pixels ) Only once in a while glare is annoying and I tip the phone and it's gone.
There are numerous hoods available as well,
b) there are dozens of ways to waterproof including as simple as a ziplock.
You are asking advice but refusing it for reasons that are not valid. Hell you've got the phone...there are numerous free nav apps - try it with say Co-Pilot as a start.
c) can't guarantee you can do what you want on the SENA 10. with twin Bluetooth feeds. Can for sure say that it works perfectly with multiple apps on the iPhone including music, phone conversation and Audible books as well as GPS directions.
If you are using audible GPS signals why are you concerned about being able to see it in the first place? There are thousands upon thousands of riders including me for years using smart phones as GPS and the apps have gotten better and better.
This poll is nearly 2 years old - you can bet the trend to smartphone use for Nav has accelerated as the apps have improved and the phones gotten brighter.
This was my first iPhone set up
and that on an iPhone 4 which I still carry as a spare. I use the X-mount now instead but with an iPhone 5. Newer models are brighter yet.