A Theory to the New EXP Changes

So to clarify for me:

2x card + 2x exp coupon + 30% exp pendant + level 2 Mercedes link + exp acquisition potion(two if on an alchemist) come out to 6.2x with one potion and 6.6x with two?

The first two get you to 4x, then the rest of them are additive, so 4.3, then 4.45, then 4.55.

It'd end up looking like this:
100% EXP Base Line
300% EXP Bonus Line
30% Item Bonus EXP
15% Elven Blessing Bonus EXP
10% Bonus EXP from the Potion

One or more of the last three lines might appear together on one line, additively.

455%, or 4.55x.

Party Play, the Guild Skill, Mercedes Link, equip bonuses, potions, and possibly Lucky Dice have their own line, so they do not multiply with the bonus EXP. HS, 1.5x/2x use buffs, family, events and the like are already multiplicative with each other rather than being on separate lines, which is what the change was in kMS.

Also, note, you cannot use two of the same potion, even if you were an alchemist. Being an alchemist only lets you use more than one of the alchemist potions in general, for example, 10% EXP and 10% Drop, or +7000 MaxHP and +10% Crit Rate, +30% Status Resistance and +24 ATT, but not 10% EXP and 10% EXP, or +7000 MaxHP and +7000 MaxHP, or +24 ATT and +24 ATT. This also extends to less effective potions of the same type. You can't use more than one HP boosting potion, so you can't do +7000 MaxHP and +5000 MaxHP, or more than one Hero potion, so you can't do +24 ATT and +18 ATT, etc.