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...

Most of 4e’s published (hardcopy) material was not Forgotten Realms and I don't think there are any specific 4e accessories for Amn. Fortunately for you, most of the 4e Living Forgotten Realms adventures are available for free online. While I don't know if there are any additional adventures based in Amn itself, as you noted there are a couple of Amnite colonies—Port Nyranzaru in Chult and Snowdown in the Moonshae Isles. Available Living Forgotten Realms adventures in the Moonshaes involving Amn and Amnian mercenaries are:

• Nature’s Wrath – Amnite pirates in the Moonshaes
• Black Gold – An Amnite agent in the Moonshaes
• Black Blood – A major encounter with Amnian mercenaries in the Moonshaes
• Black Heart – Adventure ties in with the Amnite agent from Black Gold

For more in-game historical background, there are the Forgotten Realms Campaign Setting and Player’s Guide for 3e, particularly the Shadow Thief of Amn Prestige Class in the latter.

In addition, AD&D 2e contains several Forgotten Realms accessories that directly deal with Amn, although these take place during the Time of Troubles in Faerûn before the Spellplague of 4e. Specifically,

• Lands of Intrigue – Book Two: Amn (60+ pages)
• Empires of the Sands – The first 18 pages are dedicated to Amn
• Volo’s Guide to the North – Many references to Amn are interspersed throughout

You could also use material from Baldur’s Gate 2: Shadows of Amn, the computer game and the novel, which are centered in Amn and prominently feature Spellhold.

Dragon and Dungeon magazines have some Amn-related articles. For example, "Crimmor: City of Caravans" by Ed "I am Elminster!" Greenwood in Dragon #334 (pp. 22-34) is a loquacious, detailed treatment of the "Crossroads of Amn," including a keyed map, that was nominally for 3.5e but contains almost no mechanical information. (Courtesy of @HeyICanChan. Thanks!)

There are a couple of old 4e WotC forum threads on Amn. One, entitled "Heroic Tier Adventure Ideas for Amn," is self-explanatory and the other is "Meldread's Amn," one DM's detailed version of Amn about 100 years after the Spellplague, including maps.