Ah, Ok. Multiple File/Image columns are kind of different from other columns. They function as a basic column in the fact that they actually hold data, but they also function as a computed column because they are technically an array. Computed columns can’t live on a google sheet and basic columns can’t be computed like an array. What we have here is a kind of hybrid column that’s different from any other column we’ve had before.
So, it’s kind of a weird situation. With Glide Tables, the Multiple Files/Images columns can be user specific or not, because Glide’s database can handle basic array columns like this. With Google Sheets (as well as other external data sources), they can’t handle these new column types, so they have to be user specific so the data can only live in Glide’s data storage.
I don’t know if I have a good solution for you. Your columns can only be user specific because you are using a google sheet…and you can’t use user specific columns in a form. I don’t know if Glide can or would do something to allow for these situations, or what it would involve to make it happen, but with what I know about how Glide works today, I think you are backed into to a corner. Either you migrate to a Glide Table, or you add a row and then add images/files when editing the newly added row.
Sorry, wish I had a better answer.