"Between him and his wife" is correct. You wouldn't say "It's between they". You would say "It's between them". So the objective pronoun (him) is correct and not the subjective pronoun (he).

Between your 2 lungs.

Since 27 is between 25 and 36, then √27 is between √25 and √36. So √27 is between 5 and 6, which are two consecutive numbers.

It is between Mars and Saturn.

Yes, between any two real numbers there is another real number. For example... 1.25 is between 1.2 and 1.3 1.205 is between 1.20 and 1.21 1.2005 is between 1.200 and 1.201 4.235893756385968347 is between 4.235893756385968346 and 4.235893756385968348 You can go as far as you want, there is no limit.