I'm about 80% through Deadhouse Gates, and I expect to finish this weekend, so assume my question will contain spoilers for the first two books (GotM and DG).

This is my first time through the books (listening to the audiobooks, not reading, so I have a harder time going back to reference things and I'm wary of spoiling future content by looking this up on the web/wikis). My main confusion has to do with timelines & when/how Quick Ben had time to learn all the crazy lore he knows.

In GotM we learned that Quick Ben used to be a priest of Shadow, quit that gig, and became a squad mage with the Bridgeburners. No firm timelines are given. Today I started to wonder, when did Quick Ben have time to become a priest of Shadow? When did the cult of Shadow arise? How long has the cult of Shadow been around? It's my understanding that House Shadow didn't show up in the Deck of Dragons until after Laseen became Empress, but was there a cult worshiping Shadow prior to this? It seems like most of the action of GotM takes place about a decade after Laseen took power. But it also seems like Quick Ben has been a Bridgeburner for that whole time. The lines between being a priest & a mage seem fuzzy to me still, so maybe there's less of a distinction than I think. My current impression is that the roles were sequential (priest first, then squad mage), am I wrong? Was Quick Ben filling both roles (Bridgburner mage & Shadow priest) at the same time for a while, or am I missing something?

Iskaral Pust is the only other priest of Shadow I'm aware of so far, and while he's clearly crazy [like a fox?], he also gives the impression that House Shadow has been around for a really long time. (I'm clear that the House/Hold/Warren of Shadow is potentially different from the cult of priests worshiping Shadow/Shadowthrone), I just don't understand the timelines involved. Similarly, how long was Kellanved in power? When did he conquer Seven Cities? Reading GotM, I had the impression that Quick Ben and Kalam (best bois/BFFs) grow up under Malazan rule, but reading DG, it now seems like Seven Cities was conquered during their youths.

Can anyone help me out without giving away major spoilers for the rest of the series?