Is there a way to set respawn time for single player?

There might be a LUA workaround, but it would be very complicated I think, because as far as I know SetSpawnDelay only works for AI.
You could add a timer when the player dies and don't let him spawn until the timer ends, but I have no idea how to do that/I'm not sure it's even possible.
I'm sorry that I couldn't help.

To increase spawn time for specific players, increase the PointsToUnlock of all units for the human player who died. When it's time to respawn, restore the old unlock values. This is all done client side (the host can't do it else no one would be able to spawn).
To increase spawn time for all players, have the host change the PointsToUnlock values or enable/disable the CPs at regular intervals.

To decrease spawn time for all players, you need to force spawn units at a rate faster than 15 seconds. This is slightly ugly as some people won't have time to change unit classes or pick their CP. Search GT for more info on force spawning units. It's been done a few times.

*Note to self: Add a custom user script that adds an "Enable Forced Spawning" command to the Fake Console to the list of useful, map specific FC commands that should be developed at some point once every map's CPs and path points are documented somewhere.