Soul Drinker PrC, familiar and Magical Knack Racial Trait questions.

[Mucronis]

I'm currently in a house game where we can have an evil alignment, and I've got a Cleric that's taking levels in the Prestige Class Soul Drinker.(with the DM's approval)

First off, I've read somewhere in a post here on the forum that Improved familiars special abilities increase when the familiars HD goes up, but I can't seem to find that post now. A soul Drinker gets a Cacodaemon as a familiar (and assuming it uses SD class levels as a base for when it gets better natural armor and share spells and such) I am uncertain about what it's special abilities are based off, and how to increase them as it's HD goes up.

Second, the PC has got the trait Magical Knack and is currently a cleric 5 / Soul drinker 1.
RAW the trait doesn't do anything as it's +2 trait bonus would put him above his total HD (RAI, with DM approval, i'm getting a +1 bonus as that gets me to my total HD)
At Cleric 5 / SD 2 the trait works like normal, BUT the SD PrC then gives a +1 level of existing class, but this is restricted to Spells only (spells per day, spells known and DC of spells)
Question is, how does the Trait work, same way as the SD PrC +1 caster level ? Or does the trait give all level dependent effects ? The trait is set to the Cleric class. (would it increase my channel ability ? would it effect my BAB and saves ?)
Would the trait and the SD spell level bonus stack ? so from C5 / SD2 I'd count as a C8 (+2 from trait,+1 from SD(limited to spells only)) This last way seems a bit powerful, as it would put me above my own HD in regards to spells per day and spell DC. While it would only happen in the level range of C5 / SD2 to C5 / SD6 it is still very powerful.

In advance, thank you for any and all help.

[Drejk]

Read the description of Improved Familiar feat: it states that it uses the rules for regular (which increase their intelligence and natural armor score as their master level raises and gain a handful of additional abilities too), except they don't gain ability to speak with other creatures of their kind.

Magical Knack only increases caster level for determining effects of spells, it does not grants additional spells, access to higher level spells nor does it grant class abilities (which are based on class level not caster level).

As Cleric 5/Souldrinker 2 you have spells per day as 6th level Cleric (5th Cleric, +1 from 2nd level of Souldrinker) and your caster level used when calculating effects of spells is 7th (5th CL of Cleric, +1 for Souldrinker, +2 for trait, capped at 7th for your total HD).