When sending to a non-protonmail account, you have the option to encrypt the message contents-- recipient has to open a link and enter the password. I think decryption is done in the browser in that case too (not 100% sure tho)
That is correct. The recipient can reply from that webpage as well, however you can't have a conversation there (your replies to their replies don't show up in that page, they have to get new URLs in their email).
