Responding to the original question about jacklines and tethers. My scenario is a little different, as I always have a crew. But 2/3 of that crew are grade school age. And they love to hang out on the bow, so I've thought a fair bit about keeping everyone onboard. Maybe you'll find this helpful.
Agreed that jacklines run from bow to stern cleats aren't all that helpful - at best, they'd leave you hanging in the water, and possibly fully submerged.
I ran 2 sets of jacklines. The first set is a pair in the cockpit, one on each side, running just under the lip of the seat. So we can clip in before leaving the cabin and access he whole cockpit. I used rock anchors for the aft end (see
http://sail-delmarva.blogspot.com/2012/03/bolt-hangers-strong-point-for-small.html - thanks for the suggestion, Drew). The hangers stick out a bit, but they're near our fuel tank, so not a heavily traveled area likely to skin too many knees. For the forward end I used Holt folding padeyes. They're not spec'd to quite the same strength as Wichard's, but for this application, they shouldn't ever see the huge shock load of a 5-8 foot fall from on-deck down into the water. It's much more likely that someone might lean over the side and slip, but that puts them near the end of their tether already and results in a fall of only a foot or two.
The second (and more important) set runs from anchors on the bulkheads (port and starboard) to a single padeye on the bow (in a V shape, if you can visualize that). For these, I used Wichard padeyes, rated for a 4000 lb safe working load, and all anchors are backed up by substantial G10 backing plates (recall that tension on a jackline pulls the two anchors toward one another, just like a pair of rock anchors in opposition. And a fall onto the jackline will apply a frightening load). This routing means that to stay clipped in, we have to go over the cabin top instead of along the side deck (or clip and unclip around the shrouds, which is a royal pain even with double tethers). But with such narrow side decks, I'm just fine with going over the top. It keeps everybody closer to the centerline, and (again) the jackline and short tethers should hold a crew member head-and-shoulders above the water. The downside here is that the jacklines pass over the front hatch, so you can't open the front hatch underway. I haven't found that to be a problem, but you might disagree.
For both sets, I used tubular climbing webbing, tied with water knots. Stitching would be prettier, but I trust my climbing knots more than I trust my sewing.
Tethers: We elected to go ahead and use the regular Kong ISAF tethers (single for the kids and double for the adults). There are good arguments for omitting the quick-release snap shackle (see
http://sail-delmarva.blogspot.com/2016/09/tether-shackles-no-quick-release-for-me.html). I decided that sailing a monohull, with a bow wave on the low side, made the quick-release a better bet for us. For the kids, who can't carry knives yet, there might come a time when I want to release them really fast. So, at least at the moment, we have snap shackles. But you might decide otherwise.
That system seems to work well for us. Hope it helps you a bit as you think about your own. Maybe someday I'll manage a full post with pictures; but most of the deck is torn apart right now to rebed hardware, so current pics wouldn't be clear anyway.
Resources for reading:
AAC: Start with
https://www.morganscloud.com/2016/09/29/20-things-i-have-learned-about-person-overboard-prevention/ and the various links from there. AAC has done a lot of good thinking about tethers and jackline systems. We don't go offshore, so don't have the same weather issues to think about, but I think kids can be almost as unpredictable as storms.
Drew Frye: I linked one post above. Also, be sure to read
http://sail-delmarva.blogspot.com/2016/06/too-long-tethers.html and
http://sail-delmarva.blogspot.com/2016/01/a-rant-about-jacklines-and-tethers.html. Follow the links from those to his other posts about jacklines and tethers.
