Hi, I am Dave, I will help you with this.
1- There is a bug reported by many people that IrfanView cannot be set as the default image viewer.
Right click the IrfanView icon and select 'Run as Administrator' then in IrfanView, go to Options - Set file associations - Extensions and there you can set it as default for your preferred image types.
2- With regards to uninstalling the Photos app, open PowerShell by right clicking its icon and selecting 'Run as Administrator' and run this command, then restart (not shut down) your PC to apply the change.
get-appxpackage *Microsoft.Windows.Photos* | remove-appxpackage 
