So it doesn't matter if an alternate client is run by potentially bad actors? Would you just let some random (potentially harmful) stranger keep your house safe? Of course you would not. I might be okay with a single (good) alternative. If we have more than that, then the added complexity will most likely not be worth it. I do admit that SegWit and LN add complexity but they offer advantages that make it worth it (especially LN).
Yes! That's the whole point of the thing: trustlessness. As long as there is a competition, no bad actor can enforce bad code.
Other answers later. If you have links to help me to understand your position, I'd appreciate it (instead of looking through youtube and randomly looking at presentations)