The intent is that you can only damage one cursed target (and creatures next to it) at a time

The invocation repeatedly refers to a single cursed target

As you note, the Maddening Hex eldritch invocation says (XGtE, p. 57; emphasis mine):

As a bonus action, you cause a psychic disturbance around the target cursed by your hex spell or by a warlock feature of yours [...] When you do so, you deal psychic damage to the cursed target and each creature of your choice that you can see within 5 feet of it. [...] To use this invocation, you must be able to see the cursed target, and it must be within 30 feet of you.

The feature doesn't explicitly say the damage is dealt to only one cursed target, but it repeatedly refers to the target in the singular (i.e. "the [cursed] target", or "it"). This suggests that even if you do have multiple curses active - such as a hex on one creature, and a Hexblade's Curse on another - you simply choose one of these cursed targets when you activate Maddening Hex, dealing the damage to only that creature and other creatures (of your choice) adjacent to it.

Designer intent indicates that Maddening Hex triggers off just one of the creatures you have cursed

Rules designer Jeremy Crawford answered this exact question in an unofficial tweet from November 2017:

For Maddening Hex, does the psychic damage trigger off of every target cursed by you within range, (One creature cursed by Hexblade's Curse and another cursed by the spell Hex, or the like) or only a single cursed target per turn?

Maddening Hex works on one cursed target at a time.

Crawford's unofficial ruling seems to match the wording of the feature. You may have multiple targets cursed at a time using different warlock features and/or the hex spell, but Maddening Hex only activates on one such target. As you point out, it says "the [cursed] target", singular - so even if you've cursed multiple targets, you can only use Maddening Hex to deal damage to one cursed target (and creatures adjacent to that one, of your choice) at a time.

No, Ghostly Gaze does not work that way

The Eldritch Invocation Ghostly Gaze from Xanathar's is explicit in how it works:

As an action, you gain the ability to see through solid objects to a range of 30 feet. Within that range, you have darkvision if you don’t already have it. This special sight lasts for 1 minute or until your concentration ends (as if you were concentrating on a spell). During that time, you perceive objects as ghostly, transparent images.

Nowhere in that description does it grant you the ability to see in Darkness or to see Invisible Objects.

In fact, both of those already do have mechanical options available to do so: The Eldritch Invocation Devil's Sight is what allows you to see in magical darkness and the spell See Invisibility allows you to see invisible objects. The language in those mechanics specifically allows for those interactions while the language in Ghostly Gaze does not.

There is no reason to think that such language is implicitly added. Without calling out explicitly that Ghostly Gaze grants vision in magical darkness or the ability to see invisible objects, you can not just add those mechanics to it.