Hi,
R1 - Yes you have to use several products. But you have to create product variants too.
A "attach serial" configuration is link to a serial pack. So if you want to use different configurations, you have to use several packs.
Because you can assign a pack to each variant of a product, you can have one product with a coupon selection (could be different image, template, coupon value, etc).
Otherwise, the "attach serial" plugin should be improve to get a restriction system (which is complicated to implement).
R2 - Yes, you can use a "item custom field" and display the message in the image (using "Order product .... " + the nameky of the item custom field.
R3 - AttachSerial does not support displaying formatting text in the images. So using a wysiwyg editor will just display the HTML content in the image, not the formatted text.
The text is displayed "manually" by the plugin, so it does not understand HTML.
R4 - By default no, it require specific and custom development to do that.
The "attach serial" plugin works only with HikaShop orders which have serials.
You have to work in the product page itself. Displaying something only for products which have a serial. It is quite complex.
What I can suggest you is to use an "item custom field" with a limitation of characters.
R5 - HikaSerial does not send emails, HikaShop does.
It will require a custom development in order to have an integration with a newsletter system, like Acymailing.
Mass-action won't be able to do that. The mail action is not powerful enough.
R6 - Acymailing can know when an email is open. After that I'm not sure how it could be possible to notify user A, but I guess that using plugins, it could be possible.
R7 - Using a custom plugin (which has to be develop) it is possible.
When an order is made, it is possible to check if the coupon used is a serial ; and notify the customer (so, user A) that his serial has been used.
Like R5, mass-action won't be able to do that for the same mail reason (and for the serial look up too).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.