Toby was the Seer, not Jara. I would agree with giving Toby the morphing power, but I highly doubt the other Hork-Bajir have the mental facilities to handle the morphing technology. It'd be like the Buffahuman thing all over again, only the Hork-Bajir would draw more attention since the Yeerks would know what to look for. Then there's the fact that the Yeerks found the hidden valley because they recaptured a Hork-Bajir and that didn't end well for either party.
The reason they didn't trust their friends is simple: They couldn't trust their friends. Even if they had a few close friends they absolutely knew were not controllers, what would give them the right to jump their friends into an all out war? This was the life and death process of fighting the Yeerk invasion, not selling cookies for the school band. As for the Animorphs "easily spying" on potential candidates, Tobias spied on a woman for several days without ever putting two and two together that she was the female morph of Visser Three.
Why didn't the Animorphs have the Hork-Bajir capture human controllers? Because people would notice their friends and family going missing. A former human controller wouldn't be able to go back into regular society while the Yeerks were still a threat and where are you going to keep them? The Hork-Bajir valley? The Hork-Bajir Valley wasn't meant to be the couch that the Animorphs could invite people to sleep on whenever they needed a place to crash.