That seal mounts from the outside, so yes it can be pried out and a new one installed after just removing the output flange.
Obviously, having the bearing/seal retainer housing also pulled out, makes it all much easier; especially if you have a small shop press.
There is no need to drain the oil since the max. level is at least 3" below the two bottom screws of that retainer.
If you decided to pull it then yes, use RTV-sealant to seal the (clean & dry) threads of those 4 screws.
You also need the correct thickness paper gasket for that. Reusing the old gasket + RTV on both sides may also work. Do not install without a gasket because it will deform- or even crack the housing!