"Not even I can" versus "Even I can't"

Solution 1:

Thanks Susan :) I'll try harder this time...

One more point I'd like to make on this (as I find it interesting).

If we remove the contraction in example 2, we get:

"Even I can not help them now."

and, as we recall from example 1:

"Not even I can help them now."

...meaning that whatever is going on here is merely about the placement of 'not', as can be seen in the examples.

In fact, the placement is just alternating between the subject "I" and the predicate "can help them now" containing, of course, the verb "help".

Having recently watched Noam Chomsky's 'Is the man who is tall happy' (jump to the end @ 83m 05s) I think we can see something of the same thing can be applied here, namely the moving of 'not' in the sentence to place emphasis on the subject (I) instead of the verb (help).

Solution 2:

Both phrases suggest that the speaker is normally considered powerful or capable - able to help, in your example - and if they can't do it, probably no one can.

Not even I, to me, puts more more emphasis on the I, so if you want to emphasise the speaker's position of power, use that. If you'd rather weight the emphasis on the help, use even I, but it's a subtle difference: they are interchangeable in meaning, as you said.