I think the current implementation of MIDI editing is powerful and a step above the separate window most DAWs use. As someone who is primarily a software-based producer and works with MIDI frequently, being able to select and perform operations across multiple inline rolls is a significant workflow feature for me. While I don’t see any inherent issue from allowing rolls or regions to be in a separate window, I think it would be better to focus on fixing the current usability issues people have with inline editing. From my view a separate window is a less flexible editing style and seems to only be requested because of these issues users have with the way inline editing is currently handled. I think if these issues were resolved, more people would prefer the inline editing style.
To touch on some of the suggestions in the OP:
remember the last selected track type
This is a good suggestion, I think it would be nice if we could also use modifier keys on the track list in combination with right clicks to pre-select a specific track type to add. Shift + right click for audio, Ctrl + right click for MIDI, etc.
default note velocity should be 127.
The initial default velocity should be kept the same. Many sampled instruments use relatively unaltered recordings, if the sampler is defaulted to 0db, sample playback can be well above digital zero at max velocity. This could be an issue for inexperienced users who don’t realize they have to reduce velocity or volume levels. Users should instead have the option to set their own default in the MIDI section under preferences.
make scrolling the mouse wheel scroll up and down
The main issue I see with this is that someone will run into the opposite problem, wanting to scroll the track list but being forced to scroll the roll instead and having to move their mouse over to the track list. I think a better solution would be to use modifier keys with the scroll wheel to handle scrolling the roll, similar to the way alt, shift and ctrl are currently used to expand tracks and expand/compress the timeline.
when scrolling with mousewheel to change velocity before drawing a new note, keep this new velocity value for drawing further notes
This is currently the way velocity is handled in the latest release.
double clicking inside a MIDI track creates a new 4-bar MIDI region, starting with the bar that has been double-clicked
I think this is a good suggestion, currently double-clicking opens up the track properties, which I rarely need to open, it would be nice if it was instead used for more context-sensitive editing operations, again with modifier keys to handle controlling which modes are entered, or zoom levels, along with the option to set a default.