I have a similar problem, but have solved much of it except the output.
The way I approached it was to make a FC with the fields I want to add to existing FC's. In my case I am adding a bunch of default fields and values to existing data to import the data into HAZUS-MH via their CDMS tool. Long and short of that is there's usually 15-20 fields that need to get added, then specific default values based on what type of facility i am importing.
I start the model with a merge of the existing FC and the empty-new-fields FC, then I use geoprocess Calculate Field, changing the input field and expression to a list of the added fields and their associated values.
Running the model I do not get any errors, and the app says it completed OK, but I end up with no output FC, no results. If I troubleshoot the process with a single value added to a single new field, it works and I get output, but this way I would have to daisy-chain the outputs to keep adding a new value to each new field, and get an outputted file each time. I know these can be temporary, but they would also slow the process way down, and make it hard to manage changes as the values are spread across a bunch of parameter objects, not a single list.
Anyone know a solution to why i am not getting a single output FC at the end of the model with all my new values populated into the new fields?