Thanks for playing, and I'm glad you're enjoying the game!
For Viridian Drill, digging 2 cells down instead of 1 is actually intended to be a drawback rather than a benefit. The drill has a high Power value for a relatively low crafting and repair cost, so the tradeoff is that you're forced to dig 2 cells at a time. This makes it harder to precisely target valuable cells. Since you're digging through 2 cells with each input, the -2 stamina cost is expected.
As for Winterheart, the reason you're still using 2 stamina per dig instead of 1 is that Winterheart only makes you immune to Burn, which prevents the extra stamina loss from that status effect. However it also reduces your Power by 1. In your case, it seems like you have just enough Power to dig Magma at a cost of 1 stamina. Equipping Winterheart lowers your Power below that threshold, so each dig costs 2 stamina instead. To get the full benefit of Winterheart, you'll want to have more Power than the Magma requires, so that even after the -1 Power penalty, you're still strong enough to dig it for only 1 stamina (while being immune to Burn).
Hope this helps!

