Pod,
The corner braces and strap as shown in the pictures from your link are a smart idea. The braces across the top of the corners would be easy. You could do it with scraps and wouldn’t even have to measure or cut anything if you don’t want to bother. The strap would buy you another little piece of mind but you can probably get by without it. Just make sure the frame is solidly joined and you brace the top. I have poured small things like stanchions for pitching mounds and bases on ball fields and always cinched and tied a rope around my forms just to be safe. That’s only my opinion and guess.
The lumber forms will come off. If you want to make it easier take an old paint brush and some oil and cover the edges of the lumber that will come in contact with the concrete. It’s like greasing a pan before you make brownies. 
Thank you. I think I'll do corner braces for the structural concrete layer and then maybe take them off to screed the perlcrete layer perfectly. That's really cool, I love baseball, and from what I've read on here and other sites, it's always good to tie or strap the the boards for sure. I have been getting good at buttering cake pans lately, so I will brush it with some oil. I was just about to ask how most people coat the boards; a brush makes a lot of sense.
I agree with Tony. Just to elaborate on the oil, vegetable oil would work. I skipped it because I was going to try and keep using the wood. (My upper slab 2x8 boards are currently supporting some firewood) My slab edges didn't come out the prettiest but I figured they would eventually be covered up. Should have paid more attention to them during the pour than I did.
Yes, that 20 inch lap length is just the length of the splice usually tied together with some wire. The wire is only keeping everything in place during the pour, no strength gain from the wire.
One thing I wanted to point out from ths link is the shims in places to make the top of the form level. Level is nice here, but you still get a chance for a correction in the perlcrete layer. I'd give your concrete layer 24 hours to cure. You could probably get away with less time at our current outdoor temperatures. With your plan, you could leave the form on while you se up the perlcrete form. That won't hurt anything. Put plastic over the concrete to keep the water from evaporating while the concrete cures.
That site was set up by member StoneCutter. He has built some very pretty stuff.
For wood screws, I used 3 inch deck screws I had. Pilot holes are a good idea so you don't split the boards.
1. I was just going to ask what oil is okay to use, so thank you! I will definitely oil them down a lot. Yeah, it doesn't seem really crucial to get it looking amazing when it will be covered anyway. As long as it's structural and level I'm happy. I'll make sure to make it 20" lap length if I do splice them in. It will be my first time making a rebar grid. I'm thinking of just leaving the forms up, but would a tarp over the framework be good to keep the concrete from drying out, or is it best to lay a cut piece of plastic over the concrete when it's semi cured?
Oh I didn't know it was from a member. His and all your ovens look amazing. I may do a full brick veneer over the block stand and oven.
2. I have some 2 ½" exterior screws that could work, but I think 3" would be a safer bet. So true about pilot holes, I've learned my lessons not drilling them in the past.
3. Also, my plan is to pour concrete over the firebrick and make a sloped shape to it from front to back sloping down, very much like
https://www.flickr.com/photos/climbhipa/3272605452/in/album-72157613634415857/, but I wonder the best way to install roofing onto concrete.
4. Lastly, I forgot that I'm going to put up a tall fence around the area, so I'm assuming that would cut out most wind as long as the fence is taller than the oven?