Beth,
take a look at this web site for downloading pictures to your chrome book. Its to an smartphone, but the concept is the same. just so you know, you could probably skip the chrome book and go directly to the iPhone and use a thumbdrive as your BackUp (BU) device. Maybe save you some space? Also, you shouldn't need any "file manger" software/app for your chrome book.
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you will have to look at the specs on your chrome book for the USB? interface and find the OTG adapter with some other devices to read the SD cards.
Just so you know, get the OTC device that will match your chrome books port. IE if you have USB 3.0 on your chrome book, get one for that. USB 2.0 is slower but unless your chrome book is setup for USB 3.0 its not going to help you.
as far as emailing photos, you can do that from your iPhone. If you don't know how to do it, i would ask one of your kids/nephews or take it to your store where you purchased it from and let them demo it for you.
Unless you are somewhere with free wi-fi i would throttle back a lot of photo emailing since it can chew up your data.
happy trails.