DuckDB can work on Arrow data, so I think it could coexist with DataFusion / Ballista quite well.
I am not sure whether having different execution engines for Ballista is on the roadmap, but it's certainly a possibility!
Yes, that's pretty exciting!
There is even support in Polars to execute the dataframe compute in DataFusion (as it can handle larger than memory datasets).
There is a PR from me (Daniël, committer) with for db-benchmark. For the group by benchmarks, on my machine, it is currently somewhat slower than the fastest (Polars).
Also we do support running TPC-H benchmarks. For the queries we can run, those are already finishing faster than Spark. We are planning to do more benchmarking and optimizations in the future.