Thanks for the update.
To your first question, submit records to two lists.
For the list which has been configured as the Data Source of the Editform, we use Submitform function to add records.
For the second list, we could take use of the patch function, for example, to add new records:
Patch(List2, Defaults(List2), {FieldsName:FieldValue[,FieldName:FieldValue.....]})
Thing should be noticecd is, form has two mode, Editing mode and New mode.
To add new record, we could use the formula above, but for editing record, we should use the patch function in another way, here we could identify the corresponding record in List2, then use the corresponding Value to 'Update it':
Patch(List2, first(filter(List2, condition)), {} )
Question2, to remove the list from Gallery.
Things you should know is that the Data displayed under Gallery is queried directly from datasource(here should be the sign-in list), if you would like the ID be removed from Gallery, we need to delete the corresponding item under Sign-in list.
Function which could be used to delete the item is Remove.
Question 3, to custom the DataCard, we need first un-lock the corresponding Data Card, for more details, please check:
Understand data forms in Microsoft PowerApps
Edit form and Display form controls in PowerApps
Regards,
Michael