Background: We are having a ~6PM reception with a buffet dinner. We would love to have an open bar for our guests but this is not possible due to FI's grandparents on his father's side being extremely religious and offended by alcohol. FI's own father won't even let his parent's know that he drinks once in a while. The grandmother is also a manic depressant and I don't want to do anything that will put her back into a "spell" ,as the family calls it. Therefore, we are trying to provide alcohol discretely to our guests.
EDIT: FI & I are paying for the majority of the wedding with my parents helping where they can. FI's parents are paying for the rehearsal dinner which I really appreciate.
There are actually a couple of questions rolled into one post now. Initially we were going to do assigned tables to be courteous to the guests as well as know which tables would be drinking. Now FSIL doesn't think assigned tables are a good idea. She says this bc she think people will sit wherever (I accept that this will happen to some degree). This is a wedding in a small town with the majority of FI's family being country folks. Whereas I do have some people coming from out of town, both of coworkers from a larger nearby cities. Space is also limited with nearly half of the guests being his family with about 110 guests total. I basically don't want his family to have to sit with his coworkers. And there are people that I definitely don't want to sit next to my boss.
We are serving water and lemonade from wine bottles on each 8' rectangular table. Two clear bottles with water and 2 green bottles with lemonade per table.
Idea 1) We thought we could make spiked lemonade for the tables that would be drinking and regular lemonade for those who aren't. Each bottle will be labeled as to the contents but there is still a risk of someone getting ahold of the wrong bottle (not concerned about children as there are only a few attending at a very young age, but moreso the grandparents). Also, what happens when people at a drinking table want regular lemonade... another issue.
Idea 2) Somehow provide mini bottles of liquor to those who would like it. I would probably have someone walking around serving it in order to avoid the issue of people taking handfuls at a time just to take home for later. I do understand that this will somehow happen more than likely but maybe just not to as great of an extent. This is also a more expensive option.
So with all of this in mind, what do you ladies think? Should I do the assigned tables? Which idea do you like best for the alcohol? Or do you have another suggestion; I am definitely open to ideas! 
Thanks, ladies!