I don't think you need to search very far -
See these quotes from the warlock class description (Complete Arcane P. 5-6)

WARLOCK

Born of a supernatural bloodline, a warlock seeks to master the perilous magic that suffuses his soul [...]
[...]
Adventures: Many warlocks are champions of dark and chaotic powers. Long ago, they (or in some cases, their ancestors) forged grim pacts with dangerous extraplanar powers, trading portions of their souls in exchange for supernatural power. While many warlocks have turned away from evil [...] they are still chained by the old packs through which they acquired their powers [...]
[...]
Background: Warlocks are born, not made. Some are descendants of people who trafficked with demons and devils long ago. Some seek out the dark powers as youths, [...] but a few blameless individuals are simply marked out by the supernatural forces as conduits and tools.
The exact nature of the warlock's origin is up to the player to decide;
[...]
In fact, many warlocks are created by nonevil powers - wild or fey forces that can be every bit as dangerous as demons or devils.

^{(All emphasis mine)}

AFIK, This is the source for the definition of warlock class in D&D 3.5, with Complete Mage expanding on it, including providing rules for warlock of non-fiendish origin, among other things.
At any rate, three things are stated here which are relevant to your question:

Not all warlocks have actively made a pact with a supernatural power

The exact nature of the warlock's origin is up to the player to decide.

The description provides three alternatives:

1. Your warlock made a pact with a "supernatural power" himself.
2. Your warlock's ancestors made the pact.
3. Your warlock is one of the "[few blameless individuals who] are simply marked out by the supernatural forces". He didn't choose it - they've chosen him...

So, if your warlock's is a "type 2" or "type 3" - he didn't make the pact himself.

Not all warlocks have a fiendish origin

In fact, many warlocks are created by nonevil powers - wild or fey forces that can be every bit as dangerous as demons or devils.

So, your warlock may have an elemental lord, powerful fey or even a slaadi or celestial as the source behind his power.

Every warlock owes his powers to some extraplanar or supernatural source creature

Even if your warlord didn't make a pact himself, and even if his source is not fiendish, there's no such thing as a "self empowered" warlock.

While many warlocks have turned away from evil [...] they are still chained by the old packs through which they acquired their powers.

While this have very little effect from a mechanical perspective, the warlock class is defined with this "built-in" narrative conflict or impediment - he has a would-be master - Some powerful extraplanar entity which have plans or invested interest in the warlock. Since this entity isn't as mighty as a deity, the warlock doesn't lose his powers if he deviates from his would-be master's plans, and he may even defy him outright. But, if you play a warlock, it comes as a given that you should expect some supernatural meddling in your affairs - maybe the warlock is merely observed, maybe he is not that important to that entity, and maybe it'll notice him only after he attracts its attention (by going with / against its interests or by simply becoming powerful enough to serve it as a useful tool).

So, while you are technically correct in that not all warlocks personally made a pact with a fiend, you should still work with your DM to define the source of your powers. At the very least, provide the type of that source⁽¹⁾, i.e. whether your warlock powers are fiendish / elemental / fey / celestial / etc in nature. But I'd recommend that you describe the source as an entity, with schemes and goals of its own⁽²⁾ - you'll be missing out on role-play opportunities, plot-hooks and character depth if you settle on a generic "its in my bloodline" origin.

Finally

As a side note, you state in the question that this group has a certain way of doing things, so I wonder how constructive can "smacking them with a sourcebook" be. Clearly, a healthy gaming group should be open to debate regarding rules, and perhaps even settings and campaign world elements (to some degree), but in the end, the DM calls the shots, especially when it comes to the narrative setting. So unless you can convince your DM and group to go along with your interpretation, you may have to play with them on their terms, or leave that group - no matter how many sourcebooks and splatbooks are on your side...

(1) I don't know how much leeway a DM has when running NWN2, but at a tabletop game there could be a vast difference in the way an invocation is described, and how NPCs react to such manifestations, depending on the warlock's origin - for example, a good cleric may instinctively oppose anyone who displays demonic powers, while being indifferent to someone using the same powers flavored as fey or elemental.
(2) The specifics of these schemes probably could be left to the DM to use/abuse without sharing the details and ruining the surprise for you...

My question: Is this possible?

Yes, it's possible, whether or not the fiend and the god/goddess get along. There is no RAW prohibition from the multiclass, no matter how awkward it looks.

From a purely RAW standpoint, I don't think that Warlocks can lose their powers, but would his good deity even bother with someone who sold their soul to a devil?

This depends on how the DM plays out the deity's and the fiend's role. Points to raise as you two flesh this out (a collaborative effort between you and the DM):

1. How much of a role does redemption play, a deity accepting a lost/troubled soul (bound to a devil) into the fold? The Sword Coast Adevnturer's guide supplement for D&D 5e has some material on FR deities; Baervan gets one sentence of treatment (p. 115) and Chauntea gets a few paragraphs( p. 27). For more detail, you could back up this guidance for Chauntea or Baervan with previously published material and make your case to the DM for why, or why not, either deity fits. Deities thrive on having followers.

2. Playing an internally conflicted character is rich with role playing possibility. Trying to serve the good, still dealing with evil ... that's a standard story IRL, and an element of stories from many different cultures. This would allow the link with the Pit Fiend to remain -- but there's always a price to be paid! (This could provide DM some fun, occasionally at your expense! :) )

   And would that pit fiend continue fueling their powers?

3. Why not? Here's a point of view that the Pit Fiend could take:

   "I've got my hooks into this gnome, I won't let him go. So he wants to get in with that goddess/god? Good! I can use him to cause trouble for (chosen deity), and then enjoy his suffering as he realizes that it's his fault -- due to his hunger for the power only I can give him. Muahahahahaahaa!"

   or

   "I don't like (deity), but we both have (unrelated) unfinished business with that #@!^%$ Orcus, and this gnome will serve nicely as a proxy. In the end, I profit! Muahahahaahaa!"

   You and the DM should be able to work out a deal, and some tension, that fits the campaign.

4. One last point to address as you work with your DM on this multiclass:

   How important is alignment in this campaign?

   Alignment in 5e is a bit different than in previous editions. It is more like an ideal for the character, and a matter of how the player should behave. How well the player character lives up to it, and what the rewards/penalties are, lay in the realm of DM's discretion.

With that in mind, and the two deities to choose from: Chauntea's clerics appear to require a serious commitment, and a decision on being a Pastoral or a True Shaper (the latter looks like a better fit for an adventurer), while Baervan's yoke looks like it is lighter to bear for an adventuring cleric.

Could the pit fiend and the god/goddess get along?

That would require a serious threat that both of them want to counter, something opposed to them both for different reasons ... so that you would be serving both of their interests (roughly) at the same time. Some existential threat to the world itself ... that too lies in the realm of how your DM is running and shaping the campaign.