Don’t skip this step, I think it’s fine to use by default. See this post for details.
I reckon given the relatively low resolution of your input data, upsampling would probably be a good idea. But I’m not sure it’s an absolute requirement – some users report good results without…
Always use group-averaged RFs if you can – especially for quantitative group studies.
Definitely. this is critical for connectomic analysis: if you don’t use ACT, a very significant proportion of your generated streamlines won’t reach GM regions, and hence won’t be taken into account in the construction of the connectome matrix. Some discussion about this issue in this paper.