Fixing the stupid plot
So instead of them just being to busy to build more time bridges, they can only build one because having two at the same time would could the fabric of spacetime collapse in on itself. The time bridge is built using a new element that was deposited on the earth's surface when the spaceship crashed on earth(believed to have allowed the ship to travel faster than the speed of light). The amount of this element is limited and is required to power to time bridge.
The queen aliens are actually indestructible, which is why it is actually necessary to develop an agent that can kill them. This explains why soldiers need to be sent to the future to buy people in the future time to develop a toxin, which has to be tested on one of the queen in the future (trying to take one back to the present would be too risky as it could unless the alien plague earlier than it had otherwise happens is it escaped). It is clearly established that there is only one timeline, and changes in the present do affect the future.
He sees his daughter die in the future as before, however she is unable to get a sample of the toxin to him.
It turns out that the reason that Dan left his family was because he had seen his daughter die in the future and knew that it was inevitable, which led him into a downward spiral, he could no longer bear looking at her every day knowing how she was going to die, eventually leading him to take his own life.
However it is later revealed that 6 months before he was supposed to die, Dan discovered message that his daughter encoded, with the instructions how to make the toxin, as well as a small sample of the element required to build a time bridge. He finds a scientist friend to help him replicate the toxin and build a bomb that will disperse it throughout the atmosphere. He is also able to build a time bridge to the future with the small amount of the element that will only be able to stay open for a few seconds. At the moment he is believed to have died, he travels to the future a moment after he had previously left and detonates the bomb, saving his daughter. No more of the element required to time travel remains, so he stays in the future with his daughter.
The end.