I haven no idea what it has to do with this topic, but I think sharingan was brought up because of Jiraiya vs Itachi.

- if you don't have enough chakra for a jutsu, it will fail and you might die.
Okay... and this has what to do with copying?

- you can't summon without a blood-contract
Duh, I know.

- you have to be physically able to perform copied taijutsu.
Just because someone isn't physically fit enough to perform the taijutsu, that doesn't mean that person can't copy it. Itachi could see someone do the most insane taijutsu ever and although he won't be able to do it, it doesn't necessarily mean he won't be able to copy it.