As ActiveSync doesn't supports attachments in Calendar Entries, attachments are lost when you accept a calendar entry.
I suggest to keep the invitation in the inbox as a mail - with attachments - or at least warn the user to download attachments before accept.
In the invitation, the default buttons are same as in a normal mail. To accept or decline you should go to the menu.
i suggest to change it "accept and decline" should be the default, "reply, folder, next, previuos" should go to the menu. Users very ...