The direct trains to Amersfoort take 35 minutes, run every half an hour, and cost €8.60 each way. You can check
http://ns.nl to get exact times and any possible delays just before you travel.
Amersfoort Prive is indeed easy to reach from Amersfoort station by foot. In fact, this is how I did my first several visits there.
English is pretty widely spoken across the Netherlands. The only girl I remember meeting who didn't speak English was the one I mentioned in Apeldoorn.
Aside from the prives, there isn't really a whole lot to see or do in Amersfoort as a tourist. Apeldoorn is slightly better in that regard, but neither of these towns are really tourist destinations.
One more thing regarding coffee shops: I've never had issues buying weed in either Apeldoorn or Amersfoort as a foreigner, although it's not something I do very often. Just make sure you have ID on you because they almost always ask for it.