Discipline Powers
The psion discipline ability is, much like the wizard school specialization option, worded kind of counter-intuitively. Perhaps, in a sense, even more so than school specialization, because the discipline ability doesn’t prevent you from learning any powers.
Instead, certain powers are available to psions only through the discipline ability—for example, a psion must be a shaper, that is, specialized in Metacreativity, in order to learn astral construct with their psion powers known. You can see how the astral construct power desciption lists its level as “Shaper 1,” rather than “Psion/wilder 1” as found in crystal shard’s description. You can see all of the powers from Expanded Psionics Handbook that are restricted in this fashion here; Complete Psionic has its version of those lists on pages 75 and 76.
Importantly, psions can learn other powers from those disciplines—it’s only certain, special ones that are restricted. So you have to be a shaper to learn astral construct, but any psion can learn crystal shard, even though both are Metacreativity powers.
As for dimension hop, it is not a restriced power available only to nomads (the psions specialized in Psychoportation). In fact, it’s not a psion power at all. It’s only available through the Freedom mantle, which means ardents can learn it that way. As it is not a power “restricted to other disciplines” for the psion, the psion/ardent can learn it just as easily as a single-classed ardent; nothing about the psion says otherwise.
But lets say, for the sake of example, that we were talking about astral construct, which is a restricted power for psions, only available if the psion is specialized in Metacreativity. Ardents can also learn that power, through the Creation mantle. This is a power that Expanded Psionics Handbook says “the psion cannot learn powers that are restricted to other disciplines,” which this is. Does that mean the psion/ardent cannot learn it? No, it doesn’t mean that, any more than school specialization limits what a wizard/sorcerer can learn as a sorcerer. We don’t have an explicit example of this as we do for the wizard/sorcerer example in Complete Arcane, but 1. it’s consistent with that example, and 2. it’s consistent with the fact that for the purposes of simplicity, class descriptions are written from the perspective of a single-classed character starting from 1st level.
Bottom line: Psion discipline has no effect whatsoever on what powers you can learn and manifest as an ardent, even if you are a multiclass psion/ardent.
Power Points
The answers to your questions are mostly spelled out in the Multiclass Psionic Characters section:
Multiclass Psionic Characters
If you have levels in more than one psionic class, you combine your power points from each class to make up your reserve. You can use these power points to manifest powers from any psionic class you have.
While you maintain a single reserve of power points from your class, race, and feat selections, you are still limited by the manifester level you have achieved with each power you know.
The question about combining bonus power points for each class is not explicitly addressed, but can be understood through the application of these rules: psion doesn’t just give you the base power points that come from the class, but also the bonus power points from high Intelligence. Likewise with ardent, whose power points also include the bonus power points from high Wisdom. The rules here say to combine those, so you should. Note that you still have separate manifester levels for each class! That affects how many bonus power points you get, so that in total you get
$$PP_{psion_{base}} + \tfrac{1}{2}\times Int\times ML_{psion} + PP_{ardent_{base}} + \tfrac{1}{2}\times Wis\times ML_{ardent}$$
I do seem to recall a rule saying you couldn’t get bonus power points from the same ability score twice, but I can’t find it now. That makes me suspect that it’s in Complete Psionic—it wouldn’t be a very good rule (the need to use the separate manifester level for each balances it out, no need to limit it further), and Complete Psionic is notorious for applying absolutely-unnecessary nerfs to psionic material. Be very careful with that book—it has a terrible reputation. The astral construct nerfs and Psychokinesis nerfs should be disregarded. A lot of the fluff and flavor is of very low quality, as well. The ardent is awesome, but it’s one of very, very few things in the book that is (the others are the Practiced Manifester feat and the soulbow prestige class).
