[RPG] Do these Healing items for the cleric stack when she uses her healing powers

The Healers's Brooch definitely works as it's only pre-req is the regaining of hit points.

The other two you mention are predicated on the healing keyword. Skald's Aura has this keyword so they both work.

The basic rule is to determine whether or not the power/feat/etc specifically mandates the healing keyword and whether or not the power/item/etc you are using contains that keyword. So, mark of healing requires a power to have that keyword, healer's brooch does not.

The next question would be whether or not the skald has to be spending the action for the power to take effect. I'd argue that you are still using the power even if someone else is expending the action.

There are three issues here, I think: Keywords, the two different kinds of proficiency, and permission by omission.

But before I go into those, a word: As always there are explicit features/feats/enchantments which break the rules, and that's why we call D&D an "exception-based" system: it deals in rules which apply universally unless (until) exceptions are made, so there is no need to enumerate the possible exceptions. We simply assume the rule unless told otherwise in a particular instance.

Keywords

If a power has the weapon keyword, and only if the power has the weapon keyword, does a weapon enchantment (enhancement bonuses and other features) apply to that power. Ditto with the implement keyword and implement enchantments.

Proficiency and the Proficiency Bonus

"Proficiency" means that you've had training in the use of a weapon or implement, but mechanically it means totally different things whether you're talking about a weapon or an implement.

Weapon Proficiency and the Proficiency Bonus

Proficiency with a weapon means that you can add that weapon's "proficiency bonus" to attack rolls. Only weapons have proficiency bonuses, they only apply to powers with the weapon keyword, and they have nothing to do with whether enhancement bonuses can be applied (see below for that bit).

Implements, Enhancement Bonuses, and Permission by Omission

You need to be proficient with an implement in order to add its enhancement bonus to attacks and damage with implement powers. You do not need to be proficient with a weapon in order to add its enhancement bonus to attacks and damage with weapon powers, but you don't get its proficiency bonus to the attack roll. (In either case, you can only add the enhancement bonus of one item at a time to an attack unless you have a rules exception which says otherwise.)

I arrived at this conclusion because the magic implement rules say you need to be proficient for the enhancement bonus, but the magic weapon rules don't. Permission by omission is sloppy, but has solid precedent.