Can trap monsters and defense zone create a valid field lock?
Yes, this is valid, and your logic is correct. However, it is not invincible. They can always be returned to the hand, or they can be banished, unless of course you have monsters or spells/traps to prevent this. 

The most important part of a Trap Monster that concerns this is the part that says: "This card is also treated as a Trap Card" This means that even though it is a Monster, it is also treated as a trap, hence being indestructible.

Anything that the rulebook doesn't state makes it valid.

Now... a card that stops it affects from being negated