Thevenin \$V_{th}\$ calculation problem
Your Delta-Wye transformation is wrong.
The two \$400\, m\Omega\$ resistors in your last schematics are connected in parallel. This can happen only if the current source is replaced with a short circuit. Which is wrong.

See the schematic below. (a) is the delta version and (b) is the Wye.







simulate this circuit &ndash; Schematic created using CircuitLab