Frankie Howard?

After the end titles, beyond the tunnel Dr Phibes reanimates his dead beloved wife Victoria (Caroline Munro). There is a lot of weird smoke/mist. Slowly Victoria sits up in her glass casket. The mist/fog slowly dissipates to reveal that although she has been brought back to life she now looks like Frankie Howerd! Phibes screams and screams and screams. Fade out.

No idea, maybe one of the archaeologists (maybe the one crushed in the bed or lured onto the giant scorpion.

I'm not sure he would have been a good fit...the Phibes films are quite camp enough.

"Remember, you have to make it home to get paid" (The Dogs of War)