Hi there,
Glad to help!
Given that I run a commercial site, do you recommend I completely avoid an author bio and only link (where justified) in the article body?
I would avoid linking out from the bio, that's for sure. That was / is a really common way to get a link (or another link) into a guest post and it's crazily easy to spot. If you link, link to a non-commercial property (this is being very cautious, but since guest blogging was Google's favourite thing to penalise last month, extra caution is okay!).
Some site owners have inserted bylines on my articles, eg: "By Jeepster of Jeepster.com" (followed link), making it clear it's written by a guest. Again, is that to be completely avoided?
A lot of folks have struggled with this. For the time being, I might avoid having a followed link out from a byline like that, especially if the page also says "sponsored" or "guest post" anywhere. Links from the post (which is part of the point, of course) should be okay when relevant to the text, but perhaps be cautious of "extra" followed links from both bylines and bios. There isn't much you can do if someone wants to note that the post was written by someone else, but if the post conforms to all the other quality standards of writers simply writing for each other's websites, it should be fine.
There's a reputable site in my industry that openly asks for guest writers (completely unpaid; and it's not one of those built-for-guest-posts websites). Can I safely contribute to this if my articles contain relevant links?
I would say that this sounds fairly safe, as long as the site also ticks the boxes in that list (i.e. you go through and figure out if it's been abused by SEOs and / or by the site owners for money from guest authors, which it sounds like it hasn't). Google has surprised even the tinfoil-hat-brigade with some of the stuff they've penalised or said is not okay in the last year or so, so any links out from sites that offer guest posting options aren't 100% risk-free nowadays, but I guess the idea is to calculate the risk versus reward when deciding. Sorry to not be definitive here!
I've been granted approved guest authorship to a couple of heavyweight sites in my industry, ie, I submit articles to them which are vetted at their end before going live. Is that still safe?
Same sort of thing as the other site really, although this sounds more like the "authoritative figure writing for a different authoritative site" scenario that is common in journalism, blogging, etc. We can't be sure what Google will choose to crack down on next, but they're already getting heat for trying to control how information is shared on the web and trying to crack down on knowledge being shared like this would be a further step too far, in my opinion.