1. No easy way, porbably parasitic extraction and simulation with timings could verify this, but it's actually super hard/long. I would go 0.1V IR drop, so around 2ohm resistance. Check if parasitics gets you that or if you get more, make wires more wide. Dont forget, low voltages create only one issue: Timings might not be good. 2. You are the only person that can answer that, because we dont know what are you trying to achieve. Sometimes optimal == matching requirements and overenignnering is not worth the time.