Sorry to have to break it to you, but what's going on is you. You inspect and think the heck out of things when maybe you should just plop'em in and go. Even in something as generic as a 2011 lifter (and they are generic, the one size fits all for basically every 0.904" hydraulic lifter) there are differences between manufacturers. They are subtle differences, like tiny holes around the perimeter of the metering disk, or not.

BTW I know that all 0.904 lifters aren't the same, but the 2011 is what you'll get in the majority of cases at a parts store. Eliminating part numbers is a way to decrease cost.

Now on to your barely used lifters: They're probably OK. The way to tell if they're convex is to hold two lifters foot to foot and see if they will rock slightly. Or put one on a perfectly flat surface and do the same test.
Back in the day there were some GM lifters that were manufactured with too coarse a surface on the lifter foot and we were told by Chevrolet Racing to chuck them in a lathe and lightly sand the bottom with 1000 grit paper. The grit number may not be accurate, my memory isn't sharp on that point. Another thing to look for on your barely used lifter is if any wear pattern visible is circular and concentric to the lifter body. That would be good. Any straight line from edge to edge would be bad.
It usually doesn't take a lifter long to wear once it starts. It isn't very gradual, more like abrupt.
The load the lifter carries is the second area of concern. Stock springs run like 80 - 90 lb load on the seat. This makes the pressure between lifter and lobe pretty low. Pressure is load over area, measured in psi. Seat load and open load are the two largest factors governed by the valvesprings. The cam designer controls the last factor, acceleration of the lifter. When the lifter is lifted off the base circle, it not only has the spring load but it has the load of accelerating the valvetrain. Greater lifter acceleration is the key to power and the key to wear.

Adding this together we find that if put in a stock setting your lightly used lifters will almost certainly work fine as is. The more aggressive a cam you use, the higher spring loads and inertial loads on the interface between lifter and lobe will result in higher contact pressures. So as cam gets hotter, the risk goes up. To reduce the risk you check lifter bottoms and maybe put a new slightly rough surface on them with 1000 grit paper. If you are going full bore on the cam, Delta and others will resurface the lifter ends for a small fee.

Finally, make sure the lifters rotate as the cam rotates.

Best Regards,
R.