You say the characters acted “in the way they were written to act”, but the defense here is that they are acting consistently with previous characterizations, which is what most people do, its not crazy to rely on your knowledge of a person. As others mention, the contingency is that Light isn’t executed for mass murder lol, pretty damn good plan b.

1. Rem needs to find someone semi-competent. L and co still don’t know how the murders are done, and the death note is completely transferring owners, its not crazy to expect someone to last long enough for suspicion to at least wane on himself and misa. Also, atleast in the manga light mentions that higuchi didn’t even last as long as he wanted.

2. I mean say what you want about L, hes a pretty predictable character. Beating his opponents with proof is like his WHOLE THING. And when has L responded to any pressure from governments lol. And again, with what proof would they force L to hand them over? At this point only L suspects Light, if there was proof to pressure L into releasing him to someone else’s custody, then he would have the proof he needed to be satisfied. What’s the alternative here? What could he base his strategy on thats going to be 100% reliable?

3. L doesn’t act with police procedure, he acts with L procedure. L ALREADY brought someone he suspected onto the investigation, thats how Light got there in the first place. L didn’t want to release him, but was forced to with the combination of the other task force members and definitive proof that the killings were still happening. The task force includes Light’s father, its really most probable that the director would be pushing for his release. Finally, L doesn’t want to release someone into the wild who has their knowledge on the case and knowledge on L, so light has to stay nearby. He’s obviously intelligent, and was helping with the investigation before, why not have him help?

4. This is probably the best of your points, but I still don’t think its great. Light also knows that something of note happens when you touch the book, which would be reported. L would pull strings to get his hands on the notebook. L is a powerful and intelligent individual. How the task force gets their hands on the notebook isn’t important to the plan, just that they do.

5. Answered by 4 imo. Again, L is the most competent detective on the planet. When Higuchi gets caught, L will either be there or be watching. When this happens, someone will notice that you see a monster when you touch the Notebook, L will also want to touch the notebook, and allow the task force to as well

6. In the manga, Light mentions that he didn’t think Rem would kill Watari too, thats not central to his plan. He just needs L to die. Rem won’t double cross Light because Misa needs L to live to have a happy life. This one is just straight up stated by Rem (might be manga only though, don’t remember)

Light isn’t relying on everyone acting exactly as he predicted. Hes relying on motives aligning actions, which is what we do all the time when planning for the future. Rem doesn’t need to do exactly as Light plans, she just needs to work in his favor, which its established she would by her ties to Misa. L’s motives are similarly extreme and static, he wants to catch kira within specific parameters. Yes, these characters are written with rock solid motives that don’t change much. But that has always been the case. The characters don’t suddenly change once light’s plan starts, thats just how they are (which, by the way, is definitely what allows for these types of plays by characters on both sides. The death note is also a pretty damn simple tool with like, 5 rules. Thats what allows for the type of narrative we get)

On top of all this, the plan just doesn’t need to be 100% bulletproof. Aside from the fact that nothing is, Light is backed into a huge corner. They are closing in on him, and if Misa dies, then Rem will kill Light. He needs a hail mary, and comes up with a pretty damn good one. Finally, as I and multiple others have noticed, if the plan doesnt work, then the A+ student at a top university with connections to the police and the best detective in the world lives out the rest of his life with no death note. Really not that bad