You're right, you would try WebM first. Like I said I'm not sure why you'd do both. Anything that plays VP8/9 should play h.264.

That's not a big real world issue though, but yeah if you want to provide 100% compatibility it would help a little. It's hard to justify the extra (less efficient & slower) encoding to support those few edge cases.

h.264 gives you good performance, hardware encoders/decoders, best compatibility. Most of the time just using it alone is the best value.

All that depends on how many videos you need to transcode though. If you just have a few, do all the main codecs.