No estimation needed, here is the actual wideness.
They work perfectly as overflow seating, of course there is always room for improvements, such as, like you mentioned cup holder, reclining and It would be really perfect if it had a head rest ... but even the way they are, they are OK, not the best seat in the house but work perfect as additional seating.
I'm not very handy or have the ingenuity as some of you guys... I don't see how you would incorporate cup holders to these, IMO the arm rest are not wide enough, but just like I say I don't think it can be done, someone here must be saying, sure easy does it!!!
Anyway here are the pictures, you decide.