All of the sentences are correct, except for (6).
(5) is correct as written:
(5)What you can do is (to) help her.
Quirk et al.* state:
"The complement of [this kind of sentence] is normally in the form of an infinitive clause (with or without to)." (Section 18.29, p. 1388)
Therefore (5) is is correct, either with or without to.
Other examples of this construction are:
What you need most is (to) stop whining and take responsibility
What she did was (to) alienate most of my friends
(6) is not correct. Quirk et al. state further:
"When the verb in the wh-clause has progressive aspect, however, the complement (except in the case of be going to) matches it with an -ing clause." (Section 18.29, p. 1388)
Therefore if the verb in the wh-clause were in the progressive, it would read
What you can be doing is helping her
...although that version is probably not what the speaker means. Other possible examples of this construction are
What they're doing is selecting the best players to have in reserve for performances
What I was doing was trying to make sense of your most recent messages
Quirk, Greenbaum, Leech, and Svartvik, A Comprehensive Grammar of the English Language (Longman, 1985)