This is not an overarching guide that covers everything, but the Help menu does have a search function and contains a ton of info.
You can split out Adverbs into a separate schedule. You'll need to use the Japanese Index under Learn in the menu. It has some info on the top of the page for basic usage.
You'll basically want to do the follow:
1. Add a set of words, either from some category, or you can just choose your current schedule as the source.
2. Add a filter, go to "About the word", and choose Adverbs
With the results, you can now make a new schedule.
3. For Actions, go to Schedules, and make a new one.
As to your other questions:
1. We do not yet have the linkups for N2 Kanzen, sorry! Will get it when we can!
2. Crossword app is just crosswords, nothing else. renshuu pro (it's under Support renshuu in the menu if you want a full list of what it offers) is a huge expansion of tools and content on top of renshuu. The biggest thing is that it offers many new ways to study (using sentences, writing, audio, pitch, more kanji study styles), as well as expanded content (The grammar quizzes become even more effective because the question pool grows about 4-5x for every expression, so you see the grammar in a wider variety of situations).
