>Is there a comprehensive list of shapeshifter forms somewhere,
>or is that all left to be discovered IC?

There are form lists in www.qhcf.net. The list in skill page precedes shifter revamp and doesn't have all the forms that exist now. Thus, your best bet is the diku-wiki in www.qhcf.net.

>Is cross-character play allowed at all? Can I drop items in a
>donation pit for another of my characters?

It is against the rules. Both characters involved tend to get deleted by Imms when this kind of cheating occurs. Reading the rules would be a good idea so that you don't accidentally break them.