My questions: Is this a marker of a regional Russian accent, and if so which region(s)? Or were these people more likely to be of Ukrainian heritage?
It's observed around the ukranian border (pre-crimea). Every dialect in the world is a gradient (they call them continuums) so it's very rare that they follow the country border in how immediate the change is. As such, the russian language around those borders is influenced by the ukranian language. There's also a distinct dialect called "surzhyk" from around those lands. The closer the person to the former border, the more ukranian-like they would speak, both in pronunciation and the vocabulary.
This has nothing to do with heritage, only the geographical location.
More generally, what other distinctive markers of regional accents are there, and could somebody recommend a guide?
Other than the one you've already noticed, there's none. All the information you see online regarding russian dialects is outdated. Those classifications were last used in the 50s, and were compiled in the 1910th. However the spread of radio and TV during USSR, and more so today with the internet, the russian language became very uniform, to the point that you can no longer tell someone's origin (within russia) by the way they speak, with the only exception being the southwestern regions of russia as per my description above. You also mentioned аканье, which doesn't exist anymore either.