Hi Paul:

I'm just starting a Petrel as well, and here is where my decisions, at least those made have ended up..

I would definitely add a skeg at build time , and have just cut out parts for Nick's skeg. I retrofitted a skeg to a previous build, and will now always add at build time. Feedback from reviews of Petrel paddlers seem to support this...

I went with 1/4" strips as opposed to 3/16 only because of my primitive strip cutting (small contractor saw with no real level feed table in and out of saw), and not being as skilled as Nick and some of the other builders... For the little extra wood, I'll take the extra 1/16 as contingency...

As I don't plan on building another boat (but I've made that decision 4 times before) with regard to spacers, I just use cleats, a small piece of strapping on the horizontal and vertical plane of each form.. this also provides ability to accurately align forms properly..

hope this helps..
I've always used shorter spacing joints as you mention.. though I've used butt joints in the past and they're not that bad, I find it worth the effort to mitre them (not scarf), similar to doing baseboard or crown molding