To answer your question about tethering, there are no infant seats (that I know of) that allow for tethering. Certainly the Graco Snugride seats do not even have top tethers, so it's impossible to tether them. You should install the seat with either the lower anchors (part of the LATCH system) or the vehicle seatbelt.
As for the issue with kids getting in and out of the vehicle, one solution would be to move around the seats a bit. Perhaps you could take out the 2nd row seat on the passenger's side, and then seat two children in the third row and one in the 2nd row driver's side? Then everyone could load in through the passenger's side sliding door.
If you definitely want to keep the children in the locations you specified, then you could have the older children crawl under the snugride if it is there when they go in and out of the vehicle. Although if you are putting the snugride in at the same time, simply wait until the older children load and then put the snugride in. Of course, once baby moves to a convertible, you're not going to be taking it in and out of the car. At that point, your options are to move the children into different seating locations, or have the older children crawl under the convertible. FWIW, my older kids are 6 and 8 and do sometimes crawl under the RF convertible to enter/leave our minivan.