Good to see some circuit board lurve. I've got a few WMS boards to repair and so have been thinking about some of these issues, so hope you don't mind, but here are a few thoughts. Happy to hear others' opinions.

There are a couple of different methods floating about for modifying the address select for the roms and it looks like you might have a few of these in the first incarnation of the board.  That green link wire in your first post looks a bit dodgy.  Some methods also cut traces and stick a link and or diode on the reverse of the board but I can't see it on yours.

I find the whole rom / prom changing story confusing and Clay's guide whilst great and very generously provided would benefit from some quality editing to make them more readable and usable. So much info is repeated and placed in ad hoc locations.  

Another view of the same rom stuff is presented here http://pinball.flippers.info/  and I think Leon's site talks about it also.

To me it seems the best approach is to remove all the old bodges, put J3 in, J4 out and use a 2716 game rom at IC14.  This covers most games (except Firepower).  If you do this you do not need to replaced the sockets for IC21, 22 and 26 as you'll never (I know, never say never!) use them.

The LED that has been added to your driver board is a blanking signal indicator by the look of it - do you find it any use?

As a thought what about a super cap and removal of D17 for the battery replacement?  About the same cost as batteries and a long time maybe more elegant solution.  It seems popular in the Bally world, but not in WMS which I don't understand.