What would happen if seawater was pumped into Death Valley?

many questions. Salt water does not desalinate on its own. Filling a basin with salt water would simply contaminate the groundwater of the area by salt. A saltwater basin could, over time with large flow-through of fresh water, eventually clean itself (the salt would dilute and flush over time); this would not happen in a death valley type situation. No outlet, no significant influx of fresh (rain or snow-derived) water.

The presence of water would tend to attenuate heat extremes, and evaporation would raise local humidity levels, so it is possible that the basin region could become more temperate in climate. Wind-transfer of salt water to the nearby land though, along with saltwater influx into groundwater, would be more likely to poison the surroundings against anything but salt-tolerant vegetation-that is harder to predict as a general outcome (whether the surroundings would support a viable and lush ecology).

People do not do this as a general practice because it typically doesn't help.

Death Valley already has a lot of salt in its sediment.

The water would probably be cleaned up somewhat, but it wouldn't qualify as fresh.

It wouldn't affect Lake Mead because Death Valley is below sea level and Lake Mead is not.

Below is what happens when you pump — in this case accidentally flood — fresh water into an inclosed basin. In 1905 engineers widening a section of the Colorado river flooded a canal which in turn burst its banks. The basin was filled with fresh Colorado river water and for decades served as a resort. But as the years went by and the water evaporated and was replaced, with more river water and farm runoff, the water became saltier and saltier until the lake became a festering brine hole.

Starting the process with water a thousand times more saline would not have improved the situation.

That probably wouldn't be a good idea or else people will already be doing it all over the world.

Since the salt is dissolved into the water the ground wouldn't filter it.

This would cause the levels of water to rise, but it would be salty, and most life would probably not survive since a lot of aquatic animals are either fresh water or salt water.

It wouldn't have significant effect on the temperature i don't think, it lower the average temperature of the water for a bit but that is it.

Nope or people would be doing it already

You would have a second Salton Sea. If the method of desalinization is evaporation and return as rain, then it could just as well be over the ocean.