Based on my limited understanding of the Mill I'd say (1) (simple_ PHIs are implemented by the pick-phase, (2) it's an exposed pipeline with known latencies, so you can't really have the "I want to avoid the longer latency inside a single basic block", and it's probably not worth doing that for a single mult vs. a single add, (3) for cases where it matters because the latency difference is much better, you'd use two basic blocks, and rearrange belt positions at the end.
Based on my limited understanding of the Mill I'd say (1) (simple_ PHIs are implemented by the pick-phase, (2) it's an exposed pipeline with known latencies, so you can't really have the "I want to avoid the longer latency inside a single basic block", and it's probably not worth doing that for a single mult vs. a single add, (3) for cases where it matters because the latency difference is much better, you'd use two basic blocks, and rearrange belt positions at the end.