Re: How many coins would a digi. currency need when it is used worldwide by eveyone?
Well the smallest fraction of a Bitcoin is 0.00000001 BTC which is referred to as 1 satoshi. If you don't want to use decimals, you can always use satoshi for small amounts. I personally tend to think in satoshi for amounts less than 0.001 BTC, so for example I'll refer to 0.00042000 BTC as 42k sat. This is obviously not ideal but for now it's fine for me. I'm sure the devs will think of something as Bticoin adoption rate goes up around the world. Also, the decimals are only scary/confusing in the beginning, you'll get used to it soon enough 
Well, satoshi is right now the smallest unit of BTC. However, the OP says that in the future, it may even get smaller than a satoshi and I agree with him in that.
i think they will use a sidechain for that, other wise they might screw the divisibility or you can always use another altcoin for when 1 satoshi will be worth too much, let's say 1 dollar or more
there will be always litoshi or dogetoshi, to the contrary of the popular belief, those coins are strong enough to hold their way until bitcoin will be mainstream, i doubt we will have only bitcoin as a mainstream crypto coin