I was just perusing the DH 3.0 errata and noticed an entry under skill mastery, "As it is possible to buy advances from previous ranks..."

I may have missed this line item while reading the book, unless it wasn't in there, but in the raw, do you retain access to all advances for purchase from previous ranks (provided you didn't opt out of that rank)? And there's no "penalty" for doing so, no XP increase or anything?

Yup, you maintain access to all rank tables you have passed through and can spend your XP just as you would on your current rank table.