Can you define “so much”? It recalculates every 48h, based on your baseline usage. The linked post above goes into some more detail on the calculation.
At this time, no, but feel free to add your opinion to this thread Track Use by Phase
From auto-combination/deletion/renaming, yes! Sense will only do those things if the device is marked as a guess or hasn’t been renamed from Sense’s initial detection. Find more info here: Device Detection Update: Device Updates!. If it’s still happening, regardless, shoot a message to Support.
I’d second @samwooly1’s suggestion to make use of the Device is Not On feature. You might also want to write into Support about that. Occasionally, they can do some work around conflation.
Sometimes the most obvious devices to the eye aren’t the most obvious to Sense, for a litany of reasons. Could be the time it’s used, the frequency of use, other devices that are commonly run alongside with it, something weird with the on or off transients, and more. Space heaters are a pretty good candidate for smart plugs though, especially because you gain the ability to remotely turn it off.
We’re always improving our models, so yes, I would certainly say it should improve.
Training isn’t impossible and we really hope to implement some sort of training functionality into the future, but it’s far from simple or obvious. “Hints” could certainly be useful, but correlation just wouldn’t be simple to parse out. Some quick reasons — devices overlap and mask each other, signatures look different in different contexts (including when turn on/off in rapid succession), nailing the actual start/stop of a device would be incredibly tough to do.
This kinda relates to the above. Stove is the simplest and most obvious one, with the variable draw, but even heating systems and refrigerators can look different depending on environmental factors and other concurrent devices. You can try deleting the “worst” models, or try merging all of the models together.
Ultimately, device detection can vary across homes for many reasons. 6 weeks is also fairly early in the process. Sense needs to see a lot of device cycles, and that takes time. Trust me, I feel your pain. There’s things in my home that haven’t been detected (dang 2nd leg of my water heater…), and that can be frustrating. But, I know firsthand that we’re improving all the time — I sit next to the folks who are working on those water heater models. 
Hope that helps a bit! Don’t hesitate to reach out to Support which home-specific questions either. They can look at your data and really help to parse out particular issues in your home.