E-checkin
 

So if i find a really great deal and book it do i actually have to take the flight to get the miles (on delta for example) or can i simply e-checkin over the internet and stay at home? It seems like its in everyones best interest.. I pay.. get my miles and delta can still fill the seat since im a no show.

you must take the flight.

no, you won't get the miles if you miss any of the flights. And as a courtesy* Delta will cancel the return portion of your trip (and once again no miles).

* - denotes sarcasm

Yes, you do.
No, you cannot (well, I suppose you could, but you wouldn't get credit for actually being aboard and as a result, would not get the miles credited to you).
Nope... doesn't quite work that way. The miles that truly count for anything are BIS... butt-in-seat.

You must take the flight, and please post in mileage run discussion forum next time.

You seem to be starting from a false assumption. The airlines would NOT like it if all the mileage runners started paying for status by finding a really low price per mile and just paying for as many as they want without flying to gain the status and the valuable miles / upgrade coupons / perks. Lots of people would be flying out of and into smaller airports raising the cost of flights from / to those locations due to supply/demand even if the planes are flying empty.

E