There are a few questions implied here.
Before navigating to any new layout, enter find mode first. Then do your find to the set of records you want ; or search for an empty primary key, or show all > omit records. Any of these techniques will get you to another layout quickly with no data showing.
You can also navigate to a new off screen window in the child context that has no content (nothing for FileMaker to populate) and do your scripted entry there.
But the other question; creating records in a related Table easily.
A technique I call a ghost portal is used. The child table is set with create through relationships true. If you show the child portal, hide the fields of there is no primary key - keeps the presentation clean and as the ghost record is at the end of the list of portal records, eliminates the confusion finding the end record entry place to enter data or beyond the size of the number of records shown in the portal.
Then create a single row portal from the same child table. Set the filter on that portal to false. This ends up representing the not-quite-a-record fields (the ghost record) that shows up in a portal where create through relationships is true. Now any fields from the child records in this new ghost portal will be available for entry…. but on commit, they disappear only to miraculously appear in the larger portal from the same child TO as a new record. You can even put a popover on a button in that one row portal with data entry fields in the popover window.
I often just put a plus icon in the 1 row ghost portal with the popover for the data entry fields adjacent to the larger portal. Small, Clean UI and easy to implement.
The 2 critical pieces of execution; create through relationship on and a portal filter on the second tiny portal to “false”.