I think this is a worthy discussion, but separate to the original question as to whether or not it is possible to merge observations once already uploaded. So, I moved it to its own topic. Feel free to adjust the topic title.
On Android using iNat with the default photo gallery app, to add additional photos: if you tap and hold you can then select more than one photo.
On iOS, you tap each photo you want to add and select “Done” to add more than one photo.