I ass-u-me that most of the SPAM calls I receive display a spoofed number. When I block these numbers, I also ass-u-me that the same SPAMMER can still bug me with a different spoofed number.  Are my ass-u-mptions correct?

An amusing aside. On occasion I will answer the SPAM calls adopting an exaggerated Indian accent and credulously offer whatever amount of rupees is required. I ask for their address so I can send the rupees. The most recent such instance resulted in the SPAMMER turning up his background Bollywood music.

Correct.  The spoofer can randomly generate numbers.  I've even had my number spoofed before and some woman called me and left a voicemail belligerent that I was spamming her even though I wasn't doing anything of the sort. 

I also occasionally will troll scammers.  I will reply to them and offer to give them my credit card numbers and social security number right off the bat without them even saying anything or after the "hello."  I've had them hang up on me or just not reply at all. 

  I've done the same thing: "Just a second let me get my credit card. How much do you want?"

What puzzles me is how they can conceal their real number from the carrier.