If the form doesn't include other media files, you can go to Fill Blank Form and it should be discovered and properly processed at that time.
Unfortunately that won't work. I have a fix up at https://github.com/opendatakit/collect/pull/3639 that I'm hoping will get reviewed in the next couple of days and then we'll publish Collect v1.25.2 to fix this.
The last-saved feature allows you to query the last filled record. It looks like what you want to do is to use a value from the last repeat instance as a default for the current repeat instance, is that right? This is also a very useful thing to do and doesn't require the last-saved feature. Here is a sample form that does this.
The expression in the default column is:
if(position(..) >= 1, /data/child_details[position() = position(current()/..) - 1]/child_school, '')
The position() = position(current()/..) - 1 part is what accesses the last instance of the repeat. It's in square brackets to filter the list of all repeat instances (it's an XPath predicate).
The if(position(..) >= 1, ..., ...) part is important because you don't want to try to access a repeat with index -1.
child_details should be replaced by the name of your repeat.
child_school should be replaced by the name of the question in the repeat that you are trying to use as a default.
I don't believe this can be simplified but I'm interested in hearing whether anyone else might have an alternative way of expressing this.