Your units within the transport should get their cover saves. Under Effect of Damage on Passengers (p. 80), it states the damage is treated as shooting hits:
Explodes! The unit suffers a number of Strength 4, AP- hits
equal to the number of models embarked. Resolve these as
for shooting hits, except that the controlling player allocates
any wounds caused.
When accompanied with KFF rules (p.34 w/ FAQ p. 2):
Kustom Force Field: Meks will build or scavenge powerful force
field projectors with which to protect their warty hides.
A kustom force field gives all units within 6" of the Mek a cover save of 5+.
Friendly vehicles within 6" are counted as being obscured and have a 5+ cover save.
Vehicles within 6" are treated as being obscured targets. The force field has
no effect in an assault.
If we accept all units inside a transport are within 6" of eachother, we can conclude:
- The passengers take a Str 4, AP- hit (p. 80)
- All non-passenger units and vehicles within 6" receive a Str 3, AP- hit (p. 74)
- All friendly and hostile units receive a 5+ cover save (p. 34 + FAQ p. 2)
- Friendly vehicles are obscured with 5+ cover save (p. 34 + FAQ p. 2)
- Non-friendly vehicles are obscured with no cover save.
To address the first explanation you provided directly:
Wrecked is different than exploded. Thus, the vehicle being wrecked at all is inco-wrecked.
Wrecked vehicles are left on the table and effectively become
a piece of terrain (p. 74)
Explodes! The vehicle is then removed and replaced with
scattered wreckage(area terrain) or a crater roughly the
same size as the vehicle (if you have one). (p. 74)
Therefore, you can resolve all damage and placement like this:
- Resolve Str 4, AP- hits to passenger unit
- Resolve Str 3, AP- hits to nearby models (incl. vehicles); measure 6" from transport
- Remove Transport model/add area terrain or crater
- "Surviving passengers are placed where the vehicle used to be"
- Passenger unit takes a pinning test
The rule about Deff Rollas and cover saves you cite appear to be about units being attacked by the Deff Rollas and appear irrelevant for the sake of this scenario.
7th ed. makes it about as unambiguous as possible:
The bearer, and all models within 6", receive a 5+ invulnerable save
against any shooting attacks. If the bearer is embarked in a vehicle,
then the vehicle receives a 5+ invulnerable save against any shooting