@Andrew_Li - Hello and welcome to the forums.
Your first topic is not a trivial one.
Google Forms can be configured to accept pre-filled values, extracted from a specially created URL, from within the form settings. The problem is how to populate the URL dynamically for the action URL and post method in webflow.
I don’t see that happening since you can’t embed field values in the action string. I am running through options in my head here, I am not going to code them and test them out. That would be up to you.
Is there a workaround? Yes, custom code and another form processor (if you need to capture the fields on your form) and then send to a Google form. Basically your custom code is going to listen for the submit, take the values already filled in, map them to your Google form input fields defined when you configured the form, then post the data , redirecting the user to the form that needs to be completed.
Another less complicated solution; show link to other form on success with incentive to submitter to follow the link and complete the G form. Create the link with custom code .
Maybe an altogether better approach is to use a third party form processor that asks questions one at a time. Teasing out responses. Giving the user a skip option. If they won’t give you the data now, they won’t do it later either. https://www.typeform.com/ comes to mind.
Not a native feature in webflow.
Maybe someone can come up with an alternate solution or some code to help you through. Hope what I shared helps.