For #1, you can just put #!/bin/bash at the top of the file to use Bash. Bash is still available, it’s just not the default for scripts that specify #!/bin/sh.
#2 is still currently tricky, but Rob Landley (former Busybox maintainer) is working on a full bug-for-bug compatible Bash clone called toysh which will be included in an upcoming release of Toybox[1]. Once that’s released, I’m looking forward to (hopefully) never writing a script for BusyBox ash again.