Thought I'd chime in here even if a little late.
Mousewheel scrolling between periods unfortunately had to be disabled on Windows and Linux since it was giving us too many problems with how scroll events are handled. Unexpected scrolling between views when users really only wanted to scroll the current view a bit. I have been working on getting infinite scrolling of views (so you can scroll to any period without having to jump weeks/months/etc...) but it is proving a little tricky. Hoping to have it ready soon-ish but it might take a while or never happen. If it has to be dropped then we will try and figure out a way to make the previous system work reliably.
Use of Return/Enter key for submitting the event edit is a deliberate choice, made back when we only had the inline editor, even if it might not have been the right one for everyone. In cases like this we tend to add a setting for it so that might happen as a solution to the above mentioned issue filed. It might also possibly make sense to have different defaults for the inline and dialog edit modes.
Thanks for the pointer with the illogical choice of icon for reminder, don't know how that ended up happening.
Regarding more formatting and such nice features, they are something we'd like to implement as things progress but we are a small team, mostly just a single dev for the calendar currently and we are prioritising general stability over new features at the moment. But for sure we're aiming to be the best calendar client one future day.
Thanks for using Vivaldi and taking the time to help us improve it.