I’ve read some bits about RS having resolve, but I can’t find much information other than here: Relationship Character Throughline Report
and this: http://narrativefirst.com/blog/2016/05/focusing-in-on-the-relationship-throughline
Is this unrelated to MC Resolve? How do you determine an RS resolve by how new or old it is?
I agree that the Relationship Throughline feels like the least determined, but I think that might be a feature of the theory, not a bug. If the Main Character Resolve is Change, then the Relationship Throughline will change accordingly, but if the Main Character Resolve is Steadfast, then the Impact Character Resolve must be Change, and the Relationship Throughline will change anyway. So the Relationship Throughline is going to change no matter what the resolve is.
As to how the Relationship changes, I think you’re given a lot of leeway in this regard. Neither the theory nor the program give you any information on this, other than the Issue, Problem, Symptom, etc. Is the relationship growing or declining in this story? Is it healthy or unhealthy? Do the MC and IC affect each other directly or indirectly? Is it a romantic relationship? Paternal? Companionable? Rivalrous? Err… avuncular? 
Anyways, my point is, you’ve got a lot you can do here. This is the heart of your story, which means it beats with the unique tempo you’ve given it.
*If you’re not sure what type of relationship you want to choose, this might help. Confucianism looks at five Bonds: between ruler and ruled, father and son, husband and wife, elder brother to younger brother, and friend to friend. We can describe these more laconically as political, paternal, spousal, fraternal, and congenial. Choose any of these, make them positive or negative, make them growing or declining… you’ve got a lot of options.
There are several story points relevant to the Relationship Story throughline that are not part of the storyform at this point, which might answer questions such as:
Does the relationship survive or is it broken?
Is the relationship pulling together or falling apart?
Is it a positive relationship or a negative relationship?
Etc. However, these require a subjective bias that, I suspect, are not visible from the objective bias of the current expression of the Dramatica model.
I wondered if a relationship resolve would affect how the RS Problem is seen. I thought I’d read that MC Resolve affects if the MC Problem is seen more as an actual problem (Change) or a motivation to hang onto (Steadfast). If an RS has a Problem of Feeling, I can see that being something that may bring the characters together or drive them apart… or maybe both at the same time (ex. lovers motivated to stick together by Feeling happiness, but struggling with Feeling fear of commitment due to past loss), if that’s okay to do.
I suspect this is why some movies feel down to me (and to other romantics) even though they are success/good stories–the RS ended in a split up (for lack of a better term). Alternatively, I could easily imagine a failure/bad story that would feel relatively up to me if the RS were to pop at the end. Sure, the story goal failed, and my angst remains, but we’re in a better place and that’s what’s really important! Or, less of a pop: “At least we still have each other.” “Is it that bad?” from Duplicity.