At some point, T157658: Factor out a backend from EditPage will also hopefully replace the use of a DerivativeRequest in VE and other extensions, as they will be using PageEdit directly instead of calling action=edit internally. This bug still needs to be fixed though.

I think the captcha issue was actually caused by https://gerrit.wikimedia.org/g/operations/mediawiki-config/+/66a3710620a8693b7c3666e0584a7c7d042e75a5/wmf-config/CommonSettings.php#2226 (AFAICT the code controls whether hCaptcha or FancyCaptcha is used, and the task description of T426751 also says "Sometimes I also see an hCaptcha challenge that I have to complete before I see the FancyCaptcha challenge."). The action parameter will be overwritten in the DerivativeRequest and is therefore no longer visualeditoredit (what the config code checks for) but edit. To fix this, we would need some other way to detect whether the current edit is a VE edit...