>> trying to link to several individuals in the 1870 US Federal Census<<
The US 1870 Census doesn't imply the relationship to the head of household. You must link each person individually. This is the way ALL census that DO NOT imply the relationship to head of household work.
This not a functional issue of FTMM, this issue is the user not understanding the source format.
If they had read my previous post, I stated that ALL US census 1870 and prior, function this way and why they do. It's not an issue with Ancestry, the software, the indexing. It's the format of the census itself. When a census doesn't include the relationship to the H of H, you don't want they system to guess for you, and it doesn't.