Many thanks for your clarifications Andrew - I do have a few follow-up questions.

I am indeed ultimately interested in modelling the simultaneous effect of group/diagnosis and inflammation on fxnal conn. ie. my wider hypothesis is - inflammation is indeed affecting FC in HC and MDD, but differently in both groups eg. completely different implicated subnetworks, and relationship with inflammation maybe opposite. (Hence, I thought of manipulating the contrast for both main effects instead of controlling for either...)

1) Double checking - is manipulating the interaction effect contrast under t-test, whilst controlling for group and inflammation main effects per your suggestion then testing for the above hypothesis ? 

2) Would it be redundant to test for main effect of inflammation in each group separately with NBS ?

3) If (2) is worthwhile, how would the design matrix look like given the regression model is FC ~ inflammation ? is one-sample t-test the stat test option here ?

Apologies in advance if my questions don't make sense again. Thank you for your time and input ! - Athina.