they looked ahead in faith to Christ, they didn't have the completed Bible like we do, but they had pictures, signs & types.
Abraham didn't 'get Isaac back from the dead, in a sense' then wonder what on earth the one had to do with the other, f'rinstance.
Immed after God pronounces judgment on the world cuzza Adam & Eve, & promises to send Christ in Gen 3:15, the whole killing the animal, & Adam naming Eve 'mother of all living' showed he had some type of faith in someact that God promised that would involve a baby & blood shed.
All of the prophetic promises through the OT the OT saints were not only able to cling to in faith, but understand on some level, maybe about as well as us & the Trinity

, but nevertheless, their faith wasn't a blind faith.
Were they regenerated prior? Yup. One day old Abraham was busy with his idols & God called him away from that. Abraham then moved to the next lil idol worshipping town to wait for his daddy to die. Then he moved. He was like any sinner, God draws him, but the natural inclination is to stay a sinner. (thanx God for "I"!)
You are correct that the faithful ones knew it wasn't their animal sacrifices, it was their looking ahead in faith to Christ as they shed the blood of the animal, knowing someday God would come live with them & provide their sacrifice, just like He did with Abraham. Sure, some of the ceremonies & all that got people focused on the externals, just as today some people go to church &think this is what makes them Christian, nevertheless, in spite of the abuses, it was a perfect system. OT saints had the same faith in Christ I do, that's the only way a guilty sinner is made right with God, so obviously it couldn't be anything but that, don'tcha think?