2. Well we could get into a whole discussion about precision in woodworking, but the short version is no it isn't enough but it doesn't really matter. What really matters is that like parts are exactly the same length, but what that length actually is isn't always important. When it is important you need to sneak up on the cut anyway. That's why the decimal/fraction conversions aren't that big of a deal, because if it needs to be that accurate then it's not getting cut exactly to a measurement on the first cut anyway.
4. I use table saws much more than circular saws and those blades are either 1/8" or 3/32", at least to start with. Once you resharpen the blade the kerf becomes slightly smaller. I can't easily get a metric size kerf blade for a 5/8" arbor anyway. I don't do much cutting of things out of sheet goods for the stuff I do so it's a moot point for me.
You have to account for the fact that I have to make things for customers who are going to expect measurements in imperial. So there would have to be a conversion between the two either at the beginning or end (or both) of the project. If they want a box that is 10" deep I'm going to be dealing with an oddball 254mm measurement that I can't really round off to 250 and then that has a ripple effect of every other dimension based on it being oddball.
My squares that have measurements are imperial. My calipers and dial indicator are imperial. My drill bits are imperial. My spindle sander has imperial size drums. And on and on. I do have some metric chisels. I'm also going to get hardware that comes with screw/bolt patterns that are imperial.
It's not as simple as using a different ruler and suddenly I live in a metric world.