Hi! Detailed answers below – but the short summary is, yes, both of these things should work 
While you typically define the full label set once via the "config" returned by the recipe, you can also override them on a per-task basis. The same question actually came up the other day and I shared an example here:
So depending on the example, you'd just add "config": {"labels": [...]} to the outgoing task dictionary. If no overrides are provided, the default labels are used as a fallback.
Yes, you can use the blocks UI to combine multiple interfaces. Here's an example of a custom interface consisting of ner_manual and choice (multiple choice, e.g. for text classification): Custom Interfaces · Prodigy · An annotation tool for AI, Machine Learning & NLP
In the dictionary you send out, you would then include the "text", "tokens" and "spans", as well as the "options" that can be rendered by the choice block.