The closed-back chausses are a different option. If you talk to IceFalcon, I'm sure he'll exchange it for you if you want. I called to ask him before I ordered my chausses, and he asked if I wanted open back or closed back ("stocking" style).
The open-back chausses like that are just an earlier style, before they covered the leg the entire way around in later years.
Once the lacing is done, try to have a lot of excess; that way you don't have to unlace it completely to get them off, and you can tuck the excess in the back of the padded chausses. They'll be pretty quick to put on like that; not quite as quick as the full-wrap style, but not vastly longer either.
The top part of mine was a belt loop, that was attached. It looks like the "square" bit at the top of yours should just be looped around the top of the belt, then laced down - at least the way they're made. Most SCA guys probably just throw on a regular belt and call it a day; that's IceFalcon's primary market, so it makes sense it would be built for that.
However, for us - on mine, I took off the belt loop portion, chopped it down to semi-straight, and kept the mail for other projects. Then I put a leather strip across the top, and added holes so I can point it just like I do my plate legs:
and how they work with the c-belt:
(plate legs just for sake of comparison, and because the camera phone did TERRIBLE)
You'll very likely need to trim down the part that covers the feet; make sure you have on your boots you'll be wearing when doing that. Cutting it a ring or two higher than the ground will keep it from damaging any hard surfaces (like hardwood floors and/or tile); as Sir Edward said, lace it down to keep it from flopping.
If you have Viking Leather boots with the lugged soles, you can pass the lacing between the lugs, and it should last longer since you aren't walking on it directly. If you do, you can lace it in parallel, instead of zig-zag.
