I just went and read the entire guide, and found just one significant error -
There is no point in Step 6!
Since 2.71 SE requires no firmware dump to install, you don't need to make one.
Here's a new installer, that'll take you from 2.71 to SE. I can't get it from 2.XX to 2.71 as well, as the Generic Downgrader takes up the PSP/UPDATE/EBOOT.PBP too.