I your PC had a DHCP address assigned to it by the SonicWall in the first place and now it has assigned it to some other device there is something wrong some place. If DHCP is setup correctly and working correctly, the DHCP server should NEVER assign the same IP address to two devices.
Now, what position do you hold? I can't believe that somebody in your IT department told you to statically assign your own IP address, unless you too are in IT.
Now, to try and answer your questions:
1) Well. If you have static IP address, then DHCP does not come into play. If you have configured you PC for DHCP, then at boot time and at regular intervals, your PC should be doing a lease renewal. The DHCP server controls the assignment of the address, so it (the DHCP server) should be the ultimate source and take precedence.
2) NO. At least not in my opinion. Assigning a static IP address (as long as it is NOT within the DHCP scope) will prevent this, but it also means that if there are other settings that are set via DHCP (such as default route and DNS servers) that get changed, you have to manually change your settings. If you are unaware that these change, then you will have problems. The correct solution is to figure out why the same IP address was assigned to two different hosts.