Can you use more than 1 PP to increase Temporary HP an Aegis gets from the Toughened Suit feat? If so, is the Aegis still limited to spending no more PP than their manifester level?
A triggered effect is a singular, one-time event that occurs in response to another event. This isn’t defined anywhere to my knowledge (unlike, say, Magic: The Gathering), but it’s just the reality of what the English used here (and myriad other places) means. If you want to trigger this effect again, you have to form your astral suit again—which requires you to first dismiss it, ending the previous set of temporary hit points.
Additionally, you are not allowed to spend more than 1 power point in a given triggering of this effect—nothing in the effect’s description says you can do that. It says you can spend 1 power point, not any number of power points, and it says you get a certain number of temporary hit points, not a certain number per power point spent. So the deal is always 1 power point for \$y\$ thp, not \$x\$ power points for \$x\times y\$ thp.
As for your final question, technically speaking, you are limited to spending no more power points than your manifester level only for the purpose of manifesting powers—that’s where that rule is written, and how that rule is described. This, however, is something I view as a technicality; there aren’t many open-ended non-power options for spending power points, but if you found one, I would assume it is limited to your manifester level unless the GM says otherwise. The empath medium archetype I wrote for Dreamscarred Press offered a non-power option that cost power points, and I explicitly stated that more power points than one’s manifester level could be spent for that—because I felt that without that statement, people would rule you could not and that would ruin the way the archetype works.