This book summarises current limitations and workarounds for the Yahoo Mail & Calendar facility

Calendar

Is it possible to send a calendar invitation to non-yahoo attendees ?

Yes.

Simply enter their normal email address in the invitation section.
The information will be sent to invitee, with a link for the invitee to follow.

Is it possible to send the same invitation link to all attendees ?

Yes - for Yahoo attendees only

In this case use the link at the bottom of the invitation page.

No - for non-yahoo attendees.

Each link in the invitation email is unique to the person it is sent to.
So it is not possible to send this invitation link to everyone
(e.g. after following the link and then changing the name the original name is replaced.)

Is it possible to Update a calendar entry and have the invitees automatically updated ?

Partially.

The resend of the invite contains the original event details only and not the updated event 
(e.g. if the time changes then this is not reflected in the new invite.
However, after clicking the invitation link the correct change in time details are shown

although any change in title is not shown.

Mail

Search Limitations

Emails found by the search facility finds an email in the "Drafts" folder cannot be edited.

Workaround: Open the email and copy and paste into a new email.

Draft Limitations

When a draft item is actually sent the previously saved draft item is not deleted.

Workaround: After sending the email check that it was actually sent my looking in the

"Sent" items folder and then go to "Drafts" and manually the delete the email.

IMAP support

Yahoo seems to support IMAP (as seen by tests performed with an iPhone). However, it has not been possible to date to find settings that will work.