Hi Andrew, I'll try to give you some useful pointers - although take with a pinch of salt
First of all, welcome to the
Dark Side 
At those focal lengths, the field rotation from the alt-az mount will be evident, so you were right to go for an EQ mount.
IMO, adding the CGE dovetail bar to your C9.25 is the best option going forward. I prefer the Losmandy made bars as they seem a bit more rigid, but they're also a good few more $$$.
The All Star alignment will help you hone your polar alignment (a bit). Once you've performed your normal star alignment, pick a bright star near the meridian and celestial equator. Follow the instructions on the handset. I say "a bit" I found it was good enough for visual, but I never got it quite close enough for imaging.
Since you'll be using a computer anyway, I'd recommend one of the software assisted methods for your polar alignment - the one I find the easiest is DARV in APT once you've connected up your DSLR. What DARV will want you to do is point at a star low on the east or west horizon and allow it to drive back and forth for a period of time. This will highlight any polar alignment error with a V-shape, and you iteratively adjust altitude until the star tracks back over itself. Then rinse and repeat with azimuth (pointing somewhere between zenith and the celestial equator). Once you've done it a few times, it's pretty easy to get good alignment.
f/6.3 is probably the ballpark you want to play in if you want details from smaller galaxies, but the consequence is that it will take longer exposures to get the signal over the noise terms of the sensor (that's a whole other conversation!!).
You might want to test your scopes + reducer with the camera(s) you intend to use to try to determine your usable imaging circle. With my C6 + reducer, I get wonky stars out towards the corners using a ZWO ASI1600 camera. No biggie, modern cameras have a lot of pixels and there's always a crop function in processing
How long you can image unguided with your scope/mount combo will depend on how good your polar alignment is and how well the mount tracks. Balancing your optical train may influence this also. Each mount has its own individual "characteristics" that we have to learn and adapt to.
The 224 would make an excellent guide camera (I used one for years before getting a 290 mono). As for your guiding mechanism... you'd want a bigger guide scope for the C9.25, and you'd have to fix it very rigidly to be viable to avoid flexure, which is why many end up with an off-axis guider. These are the better way to go at longer focal lengths, but are a bit of a learning curve to get going...but then imaging is full of those
Filters - consumer DSLR/mirrorless cameras already include filters akin to a UV/IR cut, but you may want to consider extra drop-ins if you have a lot of light pollution where you will image. Astronomik do a range of these for Canon (and other) cameras.
To get all this rocking and rolling, you'll want the ASCOM platform installed and the Celestron mount driver, and the relevant cables. The mount may have included a cable to connect up to the hand controller, and that will be sufficient. Some mounts allow you to connect directly to the mount bypassing the hand controller (Sky-Watcher and others), but I'm not certain about the CGEM. My AVX requires connecting the computer using the serial cable to the bottom port on the HC.
Oh, and good luck
