[RPG] rule that creatures without Con scores can not have the Regeneration special ability

I have to agree that the wording in the original rules is not a 100% explicit. It is clear that you cannot cast spells of a higher level than you have access to, but can you prepare them? The Pathfinder FAQ has a more complete answer, emphasis mine:

No. You only get the bonus spells if your class level grants you access to those spell levels. You can't even use them for lower-level spells. See page 16, Abilities and Spellcasters section: "In addition to having a high ability score, a spellcaster must be of a high enough class level to be able to cast spells of a given spell level. "

For example, a 1st-level wizard with 18 Intelligence has (according to table 1–3: Ability Modifiers and Bonus Spells) 1 bonus spell at spell levels 1, 2, and 3. However, he can only use the 1st-level bonus spell because as a 1st-level wizard he only has access to 1st-level spells (his class-based number of 2nd- and 3rd-level spells per day are "—", meaning "no access to spells of this level"). As soon as he becomes a 3rd-level wizard, he gains access to his 2nd-level spell slots and can use that bonus 2nd-level spell slot from his high Intelligence, and likewise for 3rd-level spells and bonus spells at wizard level 5.

Basically, ignore the columns for higher-level spells on table 1–3: Ability Modifiers and Bonus Spells until your class grants you access to those spell levels.

The Pathfinder FAQ is considered official in the Paizo community and is binding for Pathfinder Society (http://paizo.com/pathfinderSociety/faq#v5748eaic9qy5).

To my mind, the overriding rules here are these:

Alternate Form

The creature retains the type and subtype of its original form.

Undead Type

• No Constitution score.

You cannot have an undead that has a Constitution score, so since Alternate Form has you retain your type, you cannot gain a Constitution score. I believe this is “more specific” than the general rule that you gain the target creature’s physical ability scores when using Alternate Form.

You could argue the opposite way, however, that Alternate Form is a more specific case than the general rule that undead do not have a Constitution score. But note that this results in a living (has a Constitution score) undead (retains that type), which to me is a good enough reason to side the other way.

As for using Charisma in place of Constitution, this is a common feature for undead, but not a universal one. In some places, that privilege requires a feat (e.g. Undead Meldshaping in Magic of Incarnum). Depending on your build, I might want to tax you for it; Cha-to-everything is a fairly potent strategy as it is, and undead is a desirable type anyway. Getting to use Charisma for yet more things, and avoiding the “weakness” of the undead type’s lack of Constitution, might be problematic. But if you weren’t already pursuing a Cha-to-everything build, I’d probably give it freely.