My thoughts:
Love the new rules, definitely makes for more exciting matches!
It does drastically change the game though, personally I think games would be even more interesting/exciting if going for score was still a valid option. The ideal for me would be if they were equally valid, so a “miner” bot beat a combat bot about 50% of the time.
I don’t think lava damage should go down, but what could be changed is to a) leave a small arena in the middle until turn 400 (as others have suggested) and b) perhaps make the lava flow more slowly.
+1 for swapping worms 21 and 22. I think there might be a slight advantage for player 2 (but I’m not sure).