MacBook (12 inch Early 2015) with Dell WD15 USB-C Dock

I use my Macbook 12 2015 with the Dell WD15 at home (180w) and at work (130w) and it charges without issues and works fine with one external monitor to extend the desktop. Just note that would need to update the firmware of the WD15 to make it work fine. Before the update it didn't work well.

I highly recommend this dock station, compared to other adapters that I have on my bag, this dock station works flawless and I have multiple USB devices connected and a HDMI screen.

By what I've found out, Dell's USB-C dock is rated at 130W to 180W depending on which dock you get. Apple's own USB-C charger for your Macbook 12 inch is only rated at 29W, so you should be fine. I've hard you can even use a weaker charger, it will just charge slower.

Possibly, but docks aren't generally designed to charge other computers, just smaller devices at most.