With "Optimise iPhone Storage" mode, is it possible to tell iPhone to keep some photos/videos always locally available?
When using Optimize Storage, iCloud Photo Library does not have a feature to select photos to permanently store on device. If you’d like to send feedback to Apple, you can use www.apple.com/feedback

Personally, I’ve worked around with this limitation a few times by viewing photos I want ahead of time. Granted this won’t work for all situations. However, it worked for me when I wanted to show my grandmother in law my wedding photos. I know these steps work with up to at least 100 photos.


  
  Tap the select button in the Photos App.
  Select photos you would like to view offline.
  Tap share and choose an option such as Mail.
  Wait for the “prepare” overlay to complete.
  Cancel the share.
  


While this works, it’s not a permanent download. If your device begins to run low on space, your selected images may be replaced with small thumbnails.

There maybe other methods to share photos to another app that will permanently store photos on the device. I’ve not looked into this though.