BrightTalk does not require a download but does require the lead to have a login to the BrightTalk platform. Although it is free for them. Not sure if it meets all of your requirements or not.
Also of note, WebEx just launched their HTML5 webinar solution: https://collaborationhelp.cisco.com/article/en-us/n1pxgbz
It's a bit clunky and not maybe as sleek as other solutions, but it's another option
We are currently using ReadyTalk. And one of the reasons we use them is because there isn't the requirement to install anything on the participants side.
We use 'Readytalk' to host our webinars. It easily integrates with the marketo and doesn't require your users to download the app.