Q: 2000 Sentra GXE common issues (incl. head gasket + dipstick issue)?

It might be worth replacing it if it's bad. But, your evidence is hardly conclusive. Replacing it on a whim would be a very bad idea. Honestly, I think you need a better mechanic... one that will conclusively _find_ the problem before trying to fix it. As the saying goes, this ain't rocket science. A good mechanic should be able to nail this down quickly.

Now, first, the upper hose is always hotter. The reason is that hot coolant comes into the top of the radiator and cool water flows out the bottom. See this:

It's also possiblethat the coolant isn't flowing at all. A better (cheap) test is tofeel the radiator. The top should be hot. If no coolant is flowing, itwill likely be cool. Second, the fact that you have no heat from the heater is a major clue that you have a flow problem. If everything else is working properly, then you may have an air block in the system preventing flow. I don't know the Sentra cooling system that well to tell you if they have an issue with this - some cars do. You may have also another blockage somewhere or simply an unrelated heating valve problem.

Third, I'm concerned about where you say "my sentra won't start moving after getting stuck in stop-and-go traffic for about 5 mins. I pulled over, turned off the car and started it again, then it started driving,". What do you mean by "won't start moving" ? Did the engine stall from overheating? If so, you may have done some serious damage to the motor. Again, a mechanic should be able to check for signs of this.

Fourth, there are tests that can be run to pump up the cooling system and see if the head gasket is leaking.

Bottom line: My WAG is that someone screwed up when replacing your radiator, either by not properly bleeding the system or in some other way. You need a mechanic who knows what they are doing. If the same guy who replaced the radiator is now doing the other work, I'd find someone else. Have the new guy figure out if the first guy screwed up the radiator replacement / cooling system and fried your motor. Be sure to document everything.

I stepped on the gas, but the car wouldn't go. So I pulled over on the shoulder, I turned off the ignition, started the car again. It seemed to be fine and started going, but that's when I noticed the thermostat was all the way up. So I got off the freeway and turned off the engine.

About 2 months ago, my car had rough idle for about a day, then the next morning the car wouldn't start. I called AAA to have it towed to a shop, and the person who came with a tow truck was able to start the car by stepping on the gas pedal while turning on the ignition (after few minutes of struggling). When I brought my car to a shop, this mechanic didn't see any problems clearly, but replaced the radiator anyway, because the coolant level was noticeably low. This is a mechanic who has gotten lots of good reviews online, and I'm plannning on taking my car back to him. The mechanic who's been working (or checking) on my car this time is another mechanic. Do you think I should take the car back to the guy who replaced the radiator, or should I try to find another shop? Lastly, what do you mean by "Be sure to document everything?"

Anyhow, thanks very much for your time. I appreciate the long & very helpful response.