Ok, so I successfully managed to get a Cobalion with the desired IVs, by choosing a spread on frame 3, thus not requiring any advances.
To do the same with roamers I need one on frame 4: 4-3-2+1=0.
I saved inside the house on route 7, stepped outside to generate the roamer (Thundurus), and caught it. Calculated the IVs and found that I hit the right seed, and the IVs of the Thundurus I caught started on frame 4, my target, but the IVs were wrong; had I hit frame 5 it would have been correct.
The IVs listed in the adjacent finder for my target seed are such:
16, 12, 17,16, 29, 31, 29, 29, 31, 31
The roamer I caught on frame 4:
16/29/31/31/29/29
Contrary to the Time Finder's report, the IVs I wanted really started on frame 5, not 4. Is this a fluke or am I missing something? Do roamer spreads actually appear 1 frame later than the Time Finder reports?
I rechecked and confirmed I entered the correct MAC address, roamer box checked, and that the desired seed was supposed to be on frame 4, and when searching adjacents I make sure to move the IVs into the appropriate positions (or else no matches would be found).