Does the passage 'All Israel has a portion in the world to come' (Mishnah Sanhedrin 10:1) mean that every Jew, even if they commit sin, has a portion in the world to come simply because they are Jewish? And if so, how do we understand the Rambam in Hilkhot Teshuvah 3:6-14?

It's a large topic beyond the scope of this concise site, just in short, in the end that's correct, just about 100% of Jews go to Heaven, but some may need some kind of temporary cleansing beforehand, but then they're ready! Regarding the Rambam, see the mishna Sanhedrin (10, 2) which speaks for itself (and the Rambam obviously knows very well), which mentions only a very few individuals who have no share in the world to come. The answer is that there are several tracks of purification which prepare almost everyone for Heaven.