dashtrashtoo! I'll have a go at answering your questions:
Q1. Yes, they can switch SA on again if they want to. It's their ball and they can take it home if they want to. In fact, Dubya has declared that he has the right to withdraw the service from public use if he wants to and that he might do so in time of (unspecified) national emergency. That guy does sometimes behave like an angry 12 year old with a gun so he can't be trusted not to do something stupid to the civil availability of GPS, but it's very unlikely that his minders will let him do so because full accuracy is so useful for so many people and so many companies.
Q2. The reason why the grownup receivers use two frequencies is so that the ionospheric delay can be measured in realtime. The satellites use a fundamental frequency of 10.23MHz, which is tweaked slightly to compensate for the effects of General and Special Relativity. The L1 frequency is 154 x 10.23 Mhz and the L2 frequency is 120 x 10.23MHz. The two signals are transmitted exactly simultaneously, but arrive at different times. The delay caused by the density of the ionosphere is proportional to the square of the frequency. (L1/L2)^2 By measuring the time difference between the twosignals you can identify and eliminate the pseudorange error caused by th ionosphere.
Cheapo receivers, such as the ones fitted to cars and the small handheld jobs that you can buy for a hundred quid or so, only receive L1. To compensate for the ionsopheric delay they use a bog standard mathematical model of the ionosphere which takes into account such things as latitude and time of year. It's a bit like ISA, it dosn't tell you what the actual parameters are on the day of the race, but it does help somewhat. The model typically reduces the error by about 50% but the dual frequency trick eliminates it almost totally.
Q3. Nothing. See A2 above.
Q4. Think of the ephemeris as being a flight plan for each satellite. It lists the orbital parameters for each satellite so your receiver knows which sats to listen out for. Each satellite transmits not only its own ephemeris but also that of all of its mates. With about 30 satellites in the constellation at any one time your 12 channel receiver needs to know which ones not to bother with and it needs to know which ones produce the best geometry for a good fix. As soon as it gets lock on one satellite the receiver reads the ephemeris for the whole fleet and therefore knows when a new sat is going to rise above the horizon.
Apologies for any typos in the above. I've had a few swigs while making the brandy butter.