ETAC does a lot of things, and the output files reflect various combinations of those.

First of all, I cannot recommend the "local" option, as I was unable to get it to control the false positive rate reliably enough, and eventually I gave up on it. (It was a lot of code, and I left it in the 3dXClustSim program as a potential avenue for future work.) The local option is not vetted by publication, so you can't really cite any paper to support using it - whereas, the global ETAC option does have a paper you can cite to fight off the reviewers.

Second, the file TT.Ynlg4NLdW0M.Fred1.ETACmask.global.2sid.5perc.nii.gz is, as you say, the binary mask of voxels that survived any of the thresholding cases. And the file TT.Ynlg4NLdW0M.Fred1.ETACmaskALL.global.2sid.5perc.nii.gz shows which voxels passed each individual thresholding case -- collapsing across cases should give the same as TT.Ynlg4NLdW0M.Fred1.ETACmask.global.2sid.5perc.nii.gz

Third, running with multiple values of hpow is something I hoped would help, but does not in fact make much difference in the cases I tested - that is, the clusters that result don't differ much between different values of hpow.