Tenure possibilities for junior professorship
It depends: There are W1 professorships with a tenure track, and there are those without. If there is a tenure track, then that is usually prominently advertised, and more importantly, in writing. If there is nothing about tenure in writing, then the safe assumption is that you won't get tenure, and you will have to leave after 6 years. Verbal assurances that you might get something aren't worth much.

As to the teaching load, there are those that focus on allowing the person to qualify for a "real" professorship (W3), i.e. either as an alternative to a Habilitation or as a possibility to write your Habil. There are others were the position is used as a form of cheap labor for teaching. W1 positions with tenure track tend to fall in the first category, but there are also W1 positions without tenure track that fall in the first category. In that case, the W1 position is a stepping stone that helps getting a W3 at another university. Again, if it is a W1 aimed at allowing the inhabitant to qualify for a W3 professorship, then that is prominently mentioned and in written form. Only verbal assurances would make me worried.

If you know someone who work at a German university that you trust, you may want to let that person have a look at it. For an outsider it can be hard to read between the lines.