I just had this same problem 2 days ago when I joined Convertkit. What I did to solve the problem of having no forms show up in my drop down menu in Elementor was I created forms to go along with the forms in Elementor, for example I have 2 freebie landing pages & forms I built in Elementor previously, so I created 2 new forms in ConvertKit to correspond to those. Then in Elementor, I refreshed it and my new forms appeared and it worked, I was able to get the signup to work!
As for your 2nd question, I already had my own Thank You page created in Elementor (on my WordPress site), so what I am doing is putting the url to that thank you page in the 'redirect' section in my incentive email section for my forms in ConvertKit. After that is done, I created sequence emails (under the Automation tab) that will start sending to anyone that signs up using a specific form.
Hope this helps! I am still learning too.