I like how you said Lennar will open your walls for free, so who is going to close them back up ?
It would have depended upon what we found behind those walls
If the leak caused by Lennar (ie floor screws) - Lennar would have closed them up or paid to close them up,
A leak caused by our basement contractor (ie drywall nails) - we would have,
A leak caused by faulty line installed by the HVAC contractor - the HVAC contractor would have.
Luckily, the leak was in the coil, so no walls needed to be opened.