If a character has Ambidextrous & Two-weapon wielder (Melee and Ballistic) and is attacking in melee combat with a melee weapon + shooting with a Forearm weapon mount mounted in the same hand as the melee weapon is in, does this make the attacks -10 or +-0? Is this considered to be equal to fighting with two weapons in two hands at the same time, meaning -10, or since the attacks all come from the same hand, +-0?

The same with a Chimera pistol sword which has a one-shot Irontalon pistol mounted in the hilt: a character with the above-mentioned talents attacks with the sword and shoots with the pistol in the same round. -10 or +-0?

I thought it wouldn't even work at all since you need to use 2 different hands... but it doesn't say this anywhere you just have to wield 2 weapons which strikes out chimera psitol sword (as it is only 1 weapon as I see it, the same goes for any weapon with the exterminatus uprgade on it)

using one hand with a forearm mounting says both are in your primary hand so ambidex should be unecessary and you don't need the two weapon wielder talent to use both weapons at the same time with a -20 penalty

if having two weapon wielder, it should be -10

if having gunslinger and you use 2 pistols you should be at -0

that is as I see it

I wouldn't allow a melee and forearm mount weapon to be used in the same hand in the same round. They should be on different hands/arms and then it would be the -10 for two weapon fighting.

As for the Chimera pistol sword, it is a combi weapon which means you can use one or the other, not both in a round.