How do I find the Thevenin resistance and its corresponding voltage?
Is the 16 ohm not part of the resistance calculations anymore

You can disregard it, because the other end is open, it doesn't let any current flow through it.

do i just add r3 r5 r2 and r4 in series

Can you see that R3 is shorted? Current will hence flow through the short and R3 has no effect.

Finding Thevenin's voltage

Pretty straight forward to find \$ V_{AB} \$:

Open terminals A, B in the first circuit.
R5 and R4 go out of the picture.
That leaves you with R1, R2, R3 and two sources.
Assume all sources are ideal.
Take one source at a time and apply Superposition theorem.