tried stock messenger, hangouts, and chomp sms. they all behave the same (do not work correctly). there is definitely a bug with either 5.0, or the nexus 6, or sprint.
i have had a user tell me they received double of everything i sent, but most the time it shows "did not send" or something along those lines, tap to try again...
actually it does not in actual real-world use/performance.
https://plus.google.com/u/0/+JohnLieske/posts/hgb2457j4Tw
https://plus.google.com/u/0/+PhilNickinson/posts/7KmCmG54wQx
'photos' should do everything you need. if you don't like it, check 'quickpic'
https://play.google.com/store/apps/details?id=com.alensw.PicFolder&hl=en
yes i believe that is how it works too. if you want it to notify you visually but without a sound you just set that up in system, sound¬ification to your liking.