does it state you can replace a 0 dex/int mod only once, or for what purpose? What if the class ability allowed you to replace your dex mod with your wis mod for druid powers, or something else? According to your logic, that would preclude this. E.g. let's say you use Dilletante and Combat Virtuoso for Dex on a str-based dilletante power. Does that mean Hide Expertise ceases to function? Or that your dex mod is now useless for the rest of your abilities?
No, because nothing in the rules says anything like that.
Look, Hide Armor Expertise says "While wearing hide armor, you can gain a +2 bonus to AC
instead of using your Dexterity or Intelligence modifier to
determine your AC."
Similarly, Sentinel says, "While you are not wearing heavy armor, you can use your Constitution modifier
in place of your Dexterity or Intelligence modifier to
determine your AC."
The words "instead" and "in place of" is why these can't be used at the same time as each other. If you are already using something instead of something else, you cannot also swap that out for yet another option. The same reason a bard can't somehow take multiple power swap feats and claim that he is trading the same level 3 encounter power for four different powers from other classes - once he has swapped it, it is no longer available to be swapped for something else.
So, 100% RAI and RAW, this is how it works.
You suggest that if we accept this interpretation, it means other elements that draw on Dex/Int are somehow possibly limited. Except, as noted, both elements explicitly refer only to the use of Dex/Int for the purpose of determining your AC. Hence, no interaction with feats that deal with replacing stats for other purposes.
Again, look at the exact wording and things become clear - check Combat Virtuoso: "When you use any attack power gained through a multiclass feat, paragon multiclassing, or the half-elf ’s Dilettante racial feature, you can use Charisma for the
power’s attack rolls rather than the ability score normally used to make the attack. You still determine damage normally."
Charisma can replace Dex for the purpose of determining the power's attack rolls. Not for any other use to which Dex is put - not initiative, not AC/Reflex, not even damage. Just for this specific power's attack rolls, and nothing else.
In fact, this sets up a good comparison. Say I choose Sonnlinor's Hammer as my Dilettante power. It's a Cleric At-Will power, wisdom based, which can be used as a melee basic attack.
Let's say I take Combat Virtuoso. I can now use Cha instead of Wis for determining the power's attack roll.
Now, say I
also take Melee Training (Charisma). This
also lets me use Cha instead of Wis for determining the power's attack roll.
Can I choose to use both these feats at the same time? And add Charisma to my attack roll
twice instead of the normal approach of adding Wisdom
once?
Of course not.
Same exact thing goes for Hide Armor Expertise and Sentinels/Wardens/etc. If you have two options, either of which can be used 'in place' of something else, you can most certainly choose to use
either of those options, but you can't choose
both.