Does the Fill Factor setting affect heaps / heap tables with non-clustered indexes?
The non-clustered indexes on a heap will use the fill factor, but a heap is not an index (although it does appear in sys.indexes).

Since there is no way to reorganize a heap it can't have a fill factor. Fill factor only comes into the picture when rebuilding or creating an index; it only applies to indexes.

I believe How Forwarded Records are Read and Processed in a SQL Server Heap by Uwe Ricken may help you out. Specifically regarding how SQL Server uses PFS to determine inserts for records into the heap, and also how a non-clustered index uses the RID in relation to the heap.