For the custom install, it should be an option when you pop in the disc,
it might also be called advanced install, or choose install options.
For the getting rid of sp1 & sp2, that's the bad news. You will probably have to uninstall FSX and start with a clean install.
Another thing you could try, and this one is iffy as I have never done this with FSX, (but I have with FS9 and it worked) Is to put the FSX DVD in the drive, when it starts to run, close it. Now go to my computer and right click on the drive and choose explore. If it presents you with the contents of the disc, look for the FSX.exe and copy it to a temp folder. Now go and create a back up of the FSX.exe that is in your main FSX directory and put it somewhere safe.
When that is done, move the FSX.exe that you copied from the disc into your main FSX directory and let it overwrite the one that is there.
Then try your Accel install. If that doesn't work, then replace the new FSX.exe with your back up to put it back to the way it was.
If it does work, it should just overwrite the sp1&2 files with the ones on the Accel disc.
But as I stated, I've never tried this with FSX and I don't know if you can get access to the DVD contents the way you could with the FS9 discs. But if you can, this way should work out fine for you.

But if it doesn't, you will have to do a clean install job. :-/
Let me know how this works out for you.
