Yes, clients can backdate and submit their Check-in forms by only uploading their previous progress pictures if the check-in form includes a field to upload pictures.
The coach can do that as well if they possess the clients' picture by switching to client view in the app.
To switch to client view in the app, tap on the desired client profile, then tap the edit icon at the top right corner and choose "switch to client"
Alternatively, the coach can switch to client's view on the website by clicking "actions" in from of the client name and choosing "switch to client".