Minimum insertion for 35cm seatposts is usually 8 or 10 cm. For 40-43 it seems to be 10 cm. For the Shannon Hardcore 500mm it's 11cm, so yes I guess you can say that minimum insertion tend to go up with length. I intend to get that 50cm seatpost (sold out, but a new batch is coming from the factory in 2 weeks I've been told), and keep 15cm or more inserted. That gives me up to 35 cm extension, and I think it doesn't make sense to fit taller guys than 185cm or so there anyway due to the top tube length.
The bike frame also has a minimum insertion. It's not specified for this frame as far as I know, but it's usually 8 or 10 with a 35cm seatpost, ie it matches typical seat posts. The outer diameter of the Cannondale tandem rear seat tube is 31.8mm like on most (all?) Cannondales with 27.2mm seatpost, so I suppose one can assume that it as least as strong as any other and those usually have 10 cm minimum insertion. Going up over 35 cm seat posts it's probably wise to increase insertion so get an even longer seat post to keep more in the frame.
I guess if one is worried one could use a suspension seat post to soften the forces from any bump. I feel okay with this though, I'll use plenty of insertion and avoid the combination of tall and heavy stokers. I don't think a more massive seat clamp will help with bending forces, the seatclamp basically just avoids slippage. To spread out the bending load more insertion helps, and to soften the load peak a suspension or flexible seat post will help.
Some mountain bike frames have short seat tubes so long extension on seat posts is not unheard of. But sure, it wouldn't hurt with some better look into it than "I guess that it's fine" :-). Maybe I'll dust off my old mechanical engineering education and make some calculations, or even better find some actual mechanical engineer that can look into it.
When riding as stoker I have had the stoker bar accidentally slip in two ways - one is to turn the seatpost at the seat clamp due to too little seat clamp force (with the shim I needed to increase the torque to 7Nm, I'd like the seat post shim to make a tighter fit than it did, I may look for another one), the other is to twist the adjustable stem. With the Thorn stem there's no additional slippage interface added, as the stoker adjustable stem is replaced, but instead you get a stem-on-a-stem which means that there is some leverage which should in theory make it easier to slip. I find that the clamp of a standard forged stem is more even and better quality than of that Cannondale branded stoker stem (probably same type of quality as the original stoker bar) so it's actually less prone to slip. But I do recommend to tighten it down with a torque wrench to the max spec anyway.
I will use fixed stems whenever I have one that makes a fit. I'll get three stems in different lengths in the post tomorrow :-). It's not only for minimizing the risk of slipping, but also to make it easier to align the stoker bar, which is a bit of a pain with adjustable stems that can twist.