tmanlua, that's a bummer to have an accident like that as you are prepping for a brew day. However, here is how I would answer your questions:
1 - I would go get some more yeast. If you lost that much, it would take at least 2 step-up starters to get enough yeast to pitch in a beer of that gravity. Another option would be to get a new pack of yeast and combine it with what's in your existing starter and you may be okay with pitching rates.
2- To keep your starter viable, let it work for the full 3 days, then cover it and keep it refreigerated until you need to use it. If you have 7 days, you may try pouring off the beer in the starter (wash yeast) after 3 days in the starter and make another starter to step up your yeast count.
Either way, without multiple step-up starters you will probably be way underpitching your yeast and if it were me, I would get another package of yeast.