Getting your inseam measurement only takes minute. Assuming you have a one of those tape measure that pull out of the case that framers and handymen have. While using a big book against the wall pulled snug in the crotch will give most people a fool proof way to get a accurate measurement. You can just pull that tape measure snug into your crotch with it pointed down. Extend the tape to the floor, lock that position or hold it with your fingers so the tape doesn't slip back in and then add the tape measures case size to that. Most are.3¼ inches. It's usually stamped near the bottom edge of the case or on the back side. Way easier to do than explain!

As for your seat post slipping. Has the clamp gotten damaged from over tightening? If the gap in the band at the bold is non-existent at any point, then it's messed up or your seat post is too small. It might just be that you need to refresh the carbon paste on it so it will have some grip.

I also agree that you should get your saddle height in the ball park and probably within 1cm of where it needs to be before doing too much other stuff. Saddle height is one of the most important things to get correct on a road bike.

Also, I feel that looking at your legs to see if you have a bend in your knee can't really be done well while you are riding. Just not a good perspective from above and the knee is only extended ever so briefly before being bent again.

Edit.... oh, I thought I was in the other thread about saddle height. But this does fit the last few replies in the conversation here.