To be clear, you are allowed to have alts

But you are not allowed to use them for benefiting another account. And plundering certainly falls under that.

That's a difficult question. By the same token, it's questionable if alts are allowed to:
Do convoy (not self plunder, just convoying as normal) while in same group as the main

Join GNW

Accept weekly activity rewards

Do SA

All of those generate group fund, just like donation. So...