Sawdust from Boracare treated wood would be no more toxic that untreated wood, though you should avoid breathing in either. You should cut the wood to the final size before treating with Boracare. You do not want to waste the product on wood that will not be used and as Boracare can take up to 30 days to completely penetrate.
Boracare is only able to penetrate into raw, untreated, unstained or painted wood therefore it would not penetrate into plywood with varnish on it. You will need to sand down to the raw wood or pull up the flooring to expose the unvarnished underside and treat that.
Wood just needs to be dry before applying Bora-Care. There is no specifics on the time of day it should be applied. As long as you are not expecting rain in the next 48 hours. It can take up to 48 hours for it to dry after application. It should dry clear when mixed as directed and would not affect stain once dry.
Bora-Care can only be used on cut lumber/wood not any actual plant/tree. We would recommend to use Dominion 2L which can be applied directly where they are infesting in the stump and around/on the stump for carpenter ants. Be sure to treat around your home as well to prevent. Check out our Termite Guide for all the up to date tips on Termite prevention and control. We also have one for Carpenter ants here.
Bora-Care is usually the preferred choice if most of the wood is exposed and it is raw wood you are treating, meaning it is not painted, stained, or sealed, it is just normal wood. Bora-Care is made only for wood, and you do not have to know exactly where the termites are for it to work which is a plus. Bora-Care is sprayed evenly over all exposed surfaces of the wood and actually penetrates through the entire piece of wood. When the termites in the wood try to consume the wood after it has been treated, they ingest the Bora-Care with the wood and die. No matter where they are in the wood the Bora-Care will find them as long as you treated the exposed wood that you can see. The other main benefit is that Bora-Care stays in the wood forever, so you will not have to worry about termites or beetles infesting the wood that you treated ever again.
Bora-Care: http://www.domyownpestcontrol.com/boracare-p-100.html