Advanced Mechanics Question - Champion Availability Pool

Hello! I've seen floating around a chart with probabilities for what cost champions you will get with each roll at each level. I've also read that there's a pool of available champions, with limited numbers of each champion, so any ownership of a champion reduces said champions odds of coming up in any roll across the game. Is this correct? If so, my understanding then is that it is a 2-roll system. One, to determine which cost champion to make available, and then a 2nd to determine which champion from the pool at that cost will be made available. Is this correct? If so, when targeting specific champions, would it be the intended mechanic that we first fill our bench with other champions of the same cost prior to rolling, and then selling them back once rolling is completed? For example, let's say I have 60 gold, 4 bench slots, and I am looking for a Lucian. I see Ahri and Blitz available. Would it be beneficial to buy the Ahri and Blitz, then reroll down to say 46-48 gold, and then sell the Ahri/Blitz after? Thank you in advance for clarification on this issue.
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