I think options 2 & 3 are the best. Remember that players already look up ability scores/modifiers on a table. It's just that your group is familiar enough with the process to be able to do it from memory. If you make a simple enough table, they will be able to do the same thing once they learn it.If you look at the spoiler, are any of the methods I have more appealing to you personally?
If I were looking for a way to make this change, I'd just replace the dice in options 1 and 2 with 3d6 and finagle the correspondence between 8-15 and the 3d6 distribution until I liked it.What can you come up with that is (hopefully) simple, generates scores from 8 - 15, and averages about 12.25 or so? Any ideas?
The familiar feel of the dice might help gloss over the table change.