So DH 2 has done like all the games from BC and onwards, and let gear-creation be covered by Tech-Use.

So I wonder, what then is the purpose of the Trade Armourer skill?

But the crafting sidebar on page 68 states:

Tech-Use: This skill allows a character to create complex or technical devices such as those commonly created within the forges of Imperial worlds. With the GM’s permission, any weapon, armour, explosive, or piece of tech can be created using this skill

So why would I need anything except Tech-Use?

Further, the times listed in this table state "hours" where BC and OW state days. Mistake? Making a plasma rifle in a few hours seems ... wrong?

Well though its minor Tech-Use needs the Intelligence/Tech Aptitudes to get cheaply, while Trade (Armourer) is Intelligence/General, so price is one. Two would be that Tech-Use says " With GM's permission " which means the GM can veto the general use of the Tech-Use skill to prevent Players from building everything in creation (especially since it doesn't fit the setting, knowledge being held out of the hands of the commoners [read: you]); in comparison, Trade (Armourer) tells you exactly it can do and does not require the GM's permission (any more than normal to do anything).

For the times difference I have simply made it so that it depends on the complexity of the item being built and 'plausible' building speed.

Really Simple: Minutes.

Simple: Hours

Complex: Days

Exotic: Days+ (its similar to days but the 'days of work' on the item take about 1d4 days each)

Narrative: Baring prolonged 'downtime' its impossible and may take in-verse years to complete.

Narrative+: Retire your character and wait for the game to a skip a century or two