Cave story+: is the best ending possible in easy mode?

Correct, you can not get the best ending in Easy Mode. Kinda thought it says so at the beginning (in-game), but maybe not.

Don't read below, unless you want to get spoiled on how to pull it off.

The Booster 2.0 is unlockable on easy mode (and consequently the best ending). It's no more difficult than unlocking the Booster 2.0 on normal mode. However, if you don't have the Machine Gun, the process is a little more difficult. Take note of the red dot on the block just before the pit where you meet Booster. The red dot marks where you need to start jumping. Hold right all the way while you jump. It is difficult to pull it off, so make sure to save (there is a save point above the pit and the door you came in from) before attempting it in case you screw it up.

If your sources are right, then yes.

You must have the Booster 2.0 to get the Tow Rope and save Curly.