Does Dell have a VIB depot similar to vendors like HP and VMware? Or provide a site where you can download their controller driver VIB?
If so, you could download the VIB for the driver they are bundling in their custom ESXi image and install it from the command line on the existing host. Likely should install just fine. Vendors use the same hypervisor code and just add their drivers and optional software components.
You could use that extra T420 to install Dell's custom image, then run "esxcli software vib list" on both machines and compare what Dell is bundling vs. what you already have. Might help establish confidence in installing their driver in-place on the server you already built.