OMG you swallowed a KEFIR GRAIN!! Oh wait, that's a good thing
1) Well, as far as ingesting them, that's good because they are like swallowing a tiny 'factory' that will probably not only make it past your digestive juices in your stomach, but will allow it to find a home in your digestive tract where it will beneficially produce the needed bacterias RIGHT where they are most needed.
As a matter of fact, they are now selling something called 'probiotic pearls' in Japan (for a LOT of $$) that are basically gelatinous 'pearls' that encapsulate powdered 'probiotic' bacteria and you swallow them and it helps to get the probiotics past your stomach and into your digestive tract where it 'melts' and then puts the bacteria right where it does the most good. So you are saving yourself A LOT of money for a very high end product, and you have 'living cultures' in your grains and it's probably even more effective.
2) As far as your plumbing goes, the house's, not yours, they will not reproduce in the drain. As a matter of fact, lack of milk, hot water and then cold temps, plus chlorinated water will kill them and they will flush out like food bits from a 'sink disposal'.