JapanMS - Open-To-All Test Server; May 16th ~ 30th

No passives carry over, at all.

Well... that's stupid.

Mostly because it ruins a lot of combinations and generally invalidates a lot of the appeal.
That would mean that a Pali -> anything would lose out on:
- Final Attack?
- HP Boost?
- Shield Mastery?
- Achilles?
- Guardian?
- Divine Shield?
- ACB's mastery?

Well, there goes the entire damn point...
Mostly.

Paladin -> Hero is only useful if you can get advantage.
Shadow Partner gives more damage than Holy Lightning, and if ACA multipliers Shadow Partner then it wrecks the pomegranate out of everything.

You aren't thinking it all the way through. You can't base a combination just on a single buff.

Though the news that passive buffs don't carry over ruins a lot of the godliness, Paladins and Heroes still offer each other a butt-ton of damage enhancers.

As you said, Blast is stronger %-wise than Intrepid Slash, but that's only skimming the surface of what is a ludicrous skill.
Keep in mind Blast's other attributes: Ignore Def, extra Critical activation, raises minimum Critical %.

Now, to reiterate and give some numbers to a statement you already made, those last two traits bring Blast's average per-hit percent to 357% (if my old math is correct).
If Intrepid Slash has an average per-hit percent of 213%, then Blast is already 1.67 times as strong.

However, it's that first bit that makes Blast really sexy.
See, the one aspect that makes Dark Knights so damn valuable is that Sacrifice totally ignores defense.
Well, a Paladin -> Hero can't do that, but it can get pretty pineappleing close:
21% (Blast) + 48% (Combat Mastery) + 30% (Threaten) = 99% Ignore Defense (on anything that can be Threatened).
That'd make me cream myself.

Speaking of Threaten, I noticed your earlier math included Chance Attack.
Now, I don't know much about Dit's, but I'm pretty sure the only status effect they can inflict is stun.
Of course, Stun doesn't work on any bosses, so there's no way for a Hero or a Dit to inflict a status effect on a boss.
Having Threaten as a personal skill, however, would allow a PaliHero to constantly get Chance Attack's bonus.

That all said, isn't Holy + Lightning just barely weaker than Shadow Partner anyway?
1.37 (Holy) + .125 (Lightning) = 1.495
Shadow Partner = 1.5?

Pretty sure the .5% isn't significant when you consider everything else.
Actually, a Paladin/Hero would also have constant CO, meaning Enrage would be a 1.64% multiplier.
1.64 * 1.495 = 2.4518
1.60 * 1.5 = 2.4

So, yeah...
I think I'd forgo Flash Jump for all that.