1.
“So, whats the source of bitochon according to the Rishonim?”
I don’t know. And neither did R’ Meir Simchah!! It’s a strong point. I’ve posed his question to Talmidei Chachamim, and they admit they never thought about it. Nach is full of such Pesukim – but not in the Torah.
2.
Teva exists (as much as anything ‘exists’ – different topic). That’s the Maharal. Hashem created the world? Well, He created Teva as a system to run it. NOT that it has its own power, NOT that anything happens at any moment without Him – His Ratzon is Mekayem everything at every moment – but He did set up a system. It’s one of His messengers, like the malachim.
R’ Dessler, like many others, talks how “there is no Teva”. Perhaps there “are no malachim”‘? He’s just stressing that there is no stand alone entity called Teva, and no decision is made just because of the system. Hashem decides constantly if you should work according to Teva or there should be some other intervention (neis nigla, neis nistar). That’s what he means “it’s Him every moment”.
Same for the idea “Teva is a Neis”. It just means that everything is miraculous in the sense that Hashem could have set things up in any way he wanted, and there’s really nothing more astonishing about vinegar burning than oil. But is there a difference practically? absolutely! Extreme example: Do someone react the same if they catch a cold or are diagnosed with cancer? But to Hashem it’s all the same, there’s no Teva??? Yes – but He’s telling us 2 different things. Meaning – we understand that the fact that something is more or less dangerous according to TEVA makes a a huge practical difference according to the Torah!
3.
Your observation about Bitachon is absolutely true. See the Rmbn at the end of Ki Seitzei, about going to war – If you don’t have Bitachion, you just go home, the connection’s just not there, you can’t treat it like every other obligation!