Yes, you can download the free trial from boldminded.com, which lasts 30 days. 30 days after installation you'll be asked to enter a valid license.
Publisher saves most of its data to its own custom tables which mimic ExpressionEngine's native
exp_channel_data_field_[ID]tables. Through the use of ExpressionEngine's hooks the data form these tables are used to replace the data from the native tables when displayed on the front-end or within the control panel.
No. Since Publisher saves its data in its own tables ExpressionEngine is only aware of 1 database row for each entry.
Publisher will not translate custom field type names, help text, or Grid column titles. Publisher only supports translation of entry content.
No, Publisher does not support translation of entry comments.
This is a hard question to answer, but the first place you should check is the Requirements and third-party add-ons pages. If you know the add-on does not save its data in a custom database table, then most likely Publisher supports it. If it saves its data in a custom table, such as the Channel Images add-on, then it may not work with Publisher. If in doubt, just ask us.