More or less yes.
Your host itself acts as the common point to complete the circuit (From your daigram, negative would be bonded to host)
IIRC those hosts moh sells have a side switch, so yeah, this diagram is accurate.
Yes.
You may use your diode as that common negative point if your so inclined to do so, but since your making a 445, those diodes are case neutral. so it works out rather well if you chose that path (as this leaves you to decide if you want a case positive/negative host, the diode wont care as its electrically isolated from its case). either soldering your negative input to the case pin of the diode, or onto the side of the host/heatsink itself will work.