I have two separate tickets for back-to-back flights into and out of Amsterdam Schiphol airport on the same day next month. I will arrive from at AMS from Oslo on KLM at 11:35 with one checked bag. Then, on a separate ticket, I will fly from AMS to the U.S. on Northwest departing AMS at 14:10 same day.

First, is this enough time to retrieve the bag, go through whatever immigration queues are necessary, exit the secure area, then check-in at Northwest, go through security and immigration again, and board Northwest flight?

Second, would this be simplified greatly if I did not have a checked bag, only carry-on? In that case, would I be able to stay within the secure area and just check-in at the Northwest gate instead of at ticketing and check-in (outside the secure area)?

Any comments appreciated.

I think that should be plenty of time provided your first flight is not delayed. I would do carry-on just to avoid the hassle if possible. I found it an easy airport.

Another tip is to use the airport's website in advance to map out any transits you may need to make.

You should have enough time. If you can check-in online before you leave Norway. If you do that and have only carry on then you can ignore immigration and stay airside - check the screens for where your flight is leaving. Security is at the gate.
If you have a checked bag you will need to reclaim it, go through immigration, go up a level to departures where you will find a baggage drop (NW is handled by KLM), and then go back through passport control.
You do the same if you can't check in online before leaving Norway, but then go to a check in desk - the screens will tell you which one for your flight.
Officially you should be there three hours before your intercontinental flight, but you should be OK. Flights close about 45 minutes before departure.

there are nifty little maps of Schipol which show you the route from one gate to another and the time it will take. from memory, the longest time is only 20 mins.

unless you are tragically delayed you should be fine.

Are KLM and Northwest partners? If so, you may be able to check in for both flights. I've traveled on separate tickets several times while flying Delta and its partners and also on American Airlines & Lan.

It won't hurt to call KLM and ask if they will do it for you.

See my answer to your question in the Air Travel forum.

And yes NW/DL and KL are partners which should solve some of the issues.

Don't trust KLM to be on time. They have been getting sloppy. Security at gates for USA departures can be very long.

I suggest your second option. Carry-on only.