Hi. Just read over your project application. I am a WordPress Developer. I have a lot of experiences in connecting between WordPress with Apps like GetResponse, MailChimp, and other services are provides the API. I am sure can sync registration form in your wordpress site with getresponse list.
How I will do it ?
I will use option 1 of your solution. But you don't have to worry I will bother the HTML, wordpress cores or similar that causing malfunctions in other functions.
WordPress has a hook where after new user registered, "certain" function will be called. I can use this hook to connect/sync the "new user" data (email,name,etc) to GetResponse list. But through the API. Not HTML. The API is recommended way in GetResponse itself to make a syncronizing.
For consideration, please take a moment to visit my profile page. There are reviews and portfolio relevant with WordPress and Plugin development.
Please let me know if are interested, If possible, I'd like to discuss beforehand and know further details about this project.
Thank you, and
Regards,
Shelingholmes