Different direction
Most people think time is like a river that flows swiftly and surely in one direction. But I have seen the face of time and can tell you, they are wrong. Time is an ocean in a storm.
Personally, I dislike the ontological paradox, I simply find it annoying. Also, I find it presumptuous to assume that the actions of one being can reverberate even across one
universe, let alone arbitrarily many of them. Allow me to suggest something else.
We do away with the paradox altogether. The reason this paradox arises is because causal links are carried over when one travels back in time. This to say, when you have just travelled back in time, you can still influence your fate by interfering with your birth, with your past self, or basically with anything more significant then a grain of sand. But there is no reason to believe that said links will be carried over.
I propose that when someone / something time travels, they are essentially disconnected from the flow of time in their world for the duration of the journey, and reconnected (at a different point) when they arrive at their destination. From the moment they set foot in the past, they have begun influencing history in a different way than before. Also, as soon as they have travelled back in time, they are completely disconnected from their own pasts. Nothing, absolutely nothing can be done to erase them, once they have arrived.
Example: Enterprising company invents artificial intelligence that becomes self aware and nukes planet. In the future the resistance sends a man in the past to stop the company from inventing the AI. Unfortunately, when sent man defeats robot that was also sent back, a chip from that robot remains behind. Now, this is where the plot twists. The researchers at SkyNet LLC no longer have a reason to develop the AI from scratch. They just found a chip they can reverse engineer and get all the info they need. Unsurprisingly, what they come up with is similar to what they had before, maybe more advanced. AI takes over and nukes planet again.
So what you have when you send players back in time are free agents - agents that can act who are not, when they arrive, causally bound to their universe. It is as if an extra-planar portal were opened, and external players started influencing the world to which they came. I agree, this probably violates the conservation of mass law, and yes, the travellers will from an outside perspective appear to have just popped into existence from out of nowhere. But we are doing time travel within D&D, and it is my opinion that this can lead to a better story being told.
So, now that the characters are in the past, they can do whatever their wish. If they wanted, they could teach the tiny version of themselves to play soccer, they could convince their great-grandmothers to hook up with someone other than their great-grandfathers, or, most relevantly, they can use the knowledge they already have to stop the Psychosis.
But the characters are now under an immense deal of pressure. It is in their best interest to figure out a way to stop the Psychosis within the present. If they absolutely must travel in the past, they would have to do so only for the smallest period possible. This is because the instant they step out of the current time stream, their reality is subject to change, due to themselves. Eveyone they knew, eveyone they loved, everyone they cared about is at risk of being either slightly or vastly different. Just imagine traveling 30 years back in time. You fix the problem, but then when you get back, you no longer have a girlfriend because you never met her, your parent's child is different than you are and your parents do not know you, your home town is ruined because the orcs attacked and you were not there to stop them, etc., etc. Now imagine traveling 10000 years in the past, beating the BBEG, and then going back - the whole world could be different. You would know nothing and could relate to nothing, noone would know or be able to relate to you. So, do you choose to have a fight that you might not be able to handle, and thus risk the fate of the world, or do you go back in time to buff up, but risk losing everything you care about in the process?
So, my opinion, by which I mean no offence to previous posters, is: don't make your players fight for 10000 universes. Let them focus one one world only, and make it intensely personal for them, make them feel what it is they are fighting for.
