Does anyone know what the two pronged ships on page 31 of The Game Master's Kit are?

They're Idolator class raiders (IIRC, from their hull size in Battlefleet Gothic, they're heavy raiders, almost light frigates). There's a not-particularly-good picture of their models here , although it's hard to pick out the distinctive double-prow from that. I assure you, however, that it is of the same class.

Hmm, I guess I can just reskin the Infidel Class Raider.

Idolator Class Raider
The Idolator class raider is something of an enigma. Little is known of the whereabouts of its
construction, though rumour has it that they are built on the excommunicated forge world of Xana
II near the Eye of Terror. Idolators incorporate many systems and features which are of obviously
non-human origin, probably bought or stolen from other space-faring races, such as the Kroot
mercenaries and the Fra'al raiders. They show remarkable gunnery at long range and it is speculated
that they benefit from some kind of improved targeting system which cannot be widely replicated
by the Adeptus Mechanicus. Squadrons of Idolators are a constant menace to shipping near the Eye
of Terror and it is not uncommon for three or four of these vessels to slip through the blockade of
the Cadian Gate and wreak havoc on Imperial convoys before escaping back into the depths of the
void.
Crew: 26,000
Speed: 10
Maneuverability: 20
Detection: 15
Hull: 35
Armor: 18
Turret: 2
Ship Points: 45
Armament: Damage Strength Crit Range Mounting
Lance Battery 1d10+4 2 3 6 Prow
Macrocannons 1d10+2 4 4 9 Dorsal
Components: Jovian Pattern Class 2 Drive, Strelov 1 Warp Engine, Geller Field, Single Void Shield
Array, Combat Bridge, Vitae Pattern Life Sustainer, Voidsmen Quarters, M-100 Auger Array,
Sunsear Laser Battery, Titanforge Lance Battery
40 Space, 3 Remaining 45 Power, 1 Remaining Mounts: Dorsal, Prow
Note: The advanced targeting systems of the Idolator class raider allow it shoot accurately at long
range, without taking the standard -10 penalty.

I like Errant's write up but I was able to find official stats on page 68 of Citadel of Skulls.

Speaking of Citadel of Skulls are the two pronged ships on the bottom of page 5 also Idolator-class raiders, another raider design or just an artist taking license within their depiction? They do look tiny so I'm wondering if they may be some type of smaller attack craft, part of a squadron maybe.

What you're looking at are Hell Blade Fighters, so single pilot ships much closer to the viewer than anything else in the frame.

Those're attack craft. Specifically, the Hell Blade fighter - it's an interceptor-***-air/space superiority craft, basically. One man, roughly the same size as an Imperial Navy Lightning (smaller wingspan, but far longer airframe acting as a lifting body in atmo). Lightly armed, relatively poor maneuverability until outside of the atmosphere, but very, very fast.

Next up is the ship on page 193 of the Rogue Trader core book.

i'll take variants of the Adeptus Mechanicus's Ark Mechanicus ships, battleship-scale vessels which ply the galaxy in the Quest for Knowledge, possibly with heavy interpretation on the part of the artist, for ten.

It could be any of a number of different Mechanicus-built ships, even if built to an otherwise standard hull class, from light cruiser size or larger. They have a habit of using that non-traditional prow design. Given the apparent spacing of the hexagonal barbettes, I'd agree that it is likely a battleship, but it could be a cruiser (and the lack of a nova cannon mounted at the top of the fo'cs'l supports that possibility).

It is possible that it's an Arbites vessel, however, as they also don't use the traditional massive armour prow and fo'cs'l of most Imperial vessels, but tend to use designs in similar styles to those used by both the Mechanicus (and more typically and notably) the Astartes.
The colouring suggests Mechanicus, but the ornamentation (both that on the prow, and the large haloed skull just aft of midships) is more in keeping with another Imperial body.

It looks a whole lot like this: javascript:void(0);/*1333377198246*/

My guess would be an Admech light cruiser being used by someone else. I think the skull amidships is the mechanicus Skull and Cog symbol.

Inquisition Black Ship, perhaps?

Everything Imperial's positively dripping with skulls. The skull amidships only means it's human; the AdMech cog and skull is pretty distinctive and that isn't it.

Just my 2 thrones worth,

- V.