On the topic, I'd also like to know how the forward spawns in 5cp maps are made. I've looked around and there seems to be a lack of information on this.
I found an article on game faqs about how to set up forward spawns. Here is probably the bit that will interest you:
Give the func_respawnroom a name, and give all the info_player_teamspawn entities the same name. Let's assume they're "forward_spawn" for the func_respawnroom and "forward_spawn_point" for the info_player_teamspawn entities, for the sake of this walkthrough. Make sure they are not activated at the start of the round. Similarly, name the initial spawn and its spawnpoints "initial_spawn" and "initial_spawn_point"; you'll need all these for the next step.
Then, go to the func_capturearea's properties for the point you want to toggle the forward spawn. Specifically, the Outputs tab. Let's assume this forward spawn is for Team 1 (BLU, I think; it matches with Skin 1 for the cap base, so go by that).
For activating the forward spawn:
OnTeam1Capture, Activate "forward_spawn".
OnTeam1Capture, Activate "forward_spawn_point".
OnTeam1Capture, Deactivate "initial_spawn".
OnTeam1Capture, Deactivate "initial_spawn_point".
For deactivating the forward spawn (when Team 2 - RED, I think - captures the point):
OnTeam2Capture, Deactivate "forward_spawn".
OnTeam2Capture, Deactivate "forward_spawn_point".
OnTeam2Capture, Activate "initial_spawn".
OnTeam2Capture, Activate "initial_spawn_point".
And that should be enough to get the forward spawn working.
Here is the article if you want to read more:
https://www.gamefaqs.com/boards/437678-team-fortress-2/45488794
This might be a bit confusing, it sorta confused me at first, but I hope you can make use of it