No, Snatch does not negate the need for the Grab evolution.
There are three components at play here.
- The functional capability of automatically initiating a grapple on hit
- The "grab" special attack, which grants that capability
- The Grab evolution, which grants the grab special attack
So #3 gives you #2, and #2 gives you #1.
From the Snatch feat (emphasis mine):
The creature can start a grapple when it hits with a claw or bite attack, as though it had the grab ability.
So Snatch grants #1 from the list above, but not #2 or #3.
From the description of the Swallow Whole evolution (emphasis mine):
... The eidolon must possess the grab evolution, tied to a bite attack, to take this evolution. ...
So the Swallow Whole evolution requires #3 as prerequisite.
Snatch lets you grab without having the grab special attack. But the Swallow Whole evolution explicitly requires the Grab evolution, so you can't use Snatch to qualify for Swallow Whole.
There are no rules for this, so it's always going to be up to the DM.
Polymorph can be used on "a creature you can see", with no further restrictions. It (for all intents and purposes) automatically fails against shapechangers and creatures with 0 hit points, but that's as far as it specifies. There's one clause in Polymorph which could be argued to apply here:
The
target’s
gear
melds
into
the
new
form.
The
creature
can’t
activate,
use,
wield,
or
otherwise
benefit
from
any
of
its
equipment.
So if your DM considers a swallowed creature to be "gear" (unlikely), it would meld into the new form. Otherwise, Polymorph has no rules for this situation.
As you've noted, the rules within each creature's statblock have nothing that covers this situation either. It's noteworthy that, while each of them can only swallow creatures of a certain size, there's nothing that says what happens if a creature they've swallowed grows beyond that size. So, as a DM, I could certainly see an argument that when a creature you've swallowed becomes too big for you, the swallow ends messily.
Another point to consider about the rules for swallow within each creature's statblock are just that - within each creature's statblock. There are no global rules for swallow. Well, so, what?
The
target’s
game
statistics,
including
mental
ability
scores,
are
replaced
by
the
statistics
of
the
chosen
beast.
This means that, once Polymorph is cast, the rules that allowed the creature to have another creature inside it have ceased to apply to that creature. What does that mean? The only sensible way to deal with this paradox is to accept that the rules just don't cover this and the DM is going to have to decide what happens.
Best Answer
Your GM is likely correct.
If we look at the rules for Swallow Whole, they state:
Nowhere in there does it state you are treated like a held item, therefore you would not be polymorphed with it. Likewise, a familiar in a Familiar Satchel would not be merged with you if you polymorphed, as it is not an object.
Now if we look at Baleful Polymorph, it says:
Assuming the creature failed the second save, it would have lost the Swallow Whole ability, this would mean that you are no longer able to be swallowed by it. Thus you would be disgorged from its body.