It looks like you are coming across a known bug that we are planning to resolve in the next release of ArcGIS Online. The reference number is BUG-000107645, and you can also track the dev progress here: BUG-000107645 In the ArcGIS Online Crowdsource Reporter app when the mobile display settings uses 'S...
In the meantime, you can get around this by displaying the list first, instead of the map, although I do understand why you have the map displayed first for your use case.
Regarding your other questions:
In my opinion, if you need to do this for multiple fields, the easiest way to do this would be to export your layer to a geodatabase, download that gdb, and edit the schema of the feature class in Pro. You will likely need to add new fields with the properties you'd like, calculate across any existing data, and then delete the old fields. You can then use this updated feature class to overwrite your existing layer.
If you'd rather edit the layer directly and you need EITHER a default value OR the field to be required, the Solution Deployment Tool will allow you to add required fields, and the Add Data tool on the Data tab of the Details page of a service (under the burger menu) will allow you to add a field with a default value. The Solution Deployment Tool can then be used to either add or adjust domains on fields added through either workflow. Just like in the Pro workflow, I suggest adding the new fields with the properties you'd like, using the Calculate tool to bring across data from the old fields (click the header of the new field on the Data tab of the Details page), then delete your old fields.
In either case, keep an eye on your popup configurations. These are new fields, so you will likely need to re-do any settings such as order, aliases, and other display properties. You queries in the map may also be impacted if they reference any of the old fields.
(making these settings easier to change for people who are configuring this app is definitely on our radar!)
