The problem here is that it's going to take you a good amount of careful and tedious clean up work
For the issues concerning the excess resin and the ear cap filled with extra resin-
There's one way you can approach this. Seeing this cast w/ the ear caps attached, makes me believe the helmet is slush/roto-cast, so the inside of the ear cap is hollow. How thick, I can't tell here, but regardless, it should be okay
What I'd do, personally, is take a dremel cutting disk to it, such that you're litterally cutting through the center "slot" of the ear cap. You can see on the lower part where there is no excess resin just how deep it's supposed to go. Cutting it out with a dremel very carefully would make quick, effective work of it. The alternative really is sitting there with a needle file.
Now when I say cut out the slot, cut it out entirely until you can see through to the inside of the helmet. You will need to practice your dremel control skills here before you attempt doing this..I've been doing this since I was 14 (I'm 27 now) and gotten fairly comfortable with it that I'd do this method without much thought, but you gotta learn how to handle it, being conscious of where the blade is, making sure that it's not going to "jump," and most importantly, knowing where the dremel's "coilet nut" (the part of the dremel that you loosen and tighten to secure a bit) is when making cuts. That piece alone will ruin your day if you're not paying attention to where it's at. It's spinning with the blade too and will just as easily tear up another area while you're focused on what you're intending to cut.
Now that I've explained that, you make the cut so that whole slot is void of material. You'll then need to smooth and clean the interior- I'd suggest starting out with low-grit sandpaper (80 or less) just to get the interior walls clean and flushed. To clean up the rounded top and squared bottom, you should use needle files, and then follow up with progressively higher-grit sand paper
Once that slot is as neat as you can get it, go from the inside of the helmet, and make the sides as even as possible. Now, depending on what you're using the helmet for, you may be able to just leave it open and back it with black mesh (to aid with helmet ventilation) or, as I would, back it with a piece of thin plastic, like sintra OR preferably, styrene ("no parking signs" at home depot are made of thin styrene. Dirt cheap source FYI).
If you followed what I laid out properly, and back it with the styrene, you should have a clean looking ear cap now.
For the key slots. I hate to say it, but for the build up of excess resin, just gonna have to stick it out with a need file and go to work. Carefully get at it. You could probably get away with a using a dremel sanding drum, but you have to be extra careful.
For the key slots, I've always made a horizontal cut with a dremel, making it wide enough for my needle files to get through. Key slots are VERY time consuming that way, but you have much more control and can ensure nice, sharp lines. If you do this with needle files, ensure that you're keeping the files perfectly at 90 degrees to the surface, cause you'll easily grind into them a fun, angled surface.
Hope this helps
-TC