In terms of sankharas, Sanna and vedanna are citta sankharas and as usual, sankharas stems from ignorance (like asavas) and the rest of what is conditioned stems from contact and dukkha form craving.
Since sanna and vedana are citta sankharas, qualifying them of physiological is not a good idea. Same thing with trying to separate them. TYpically, the higher realm have vedana and sanna, without having the 4 elements, meaning without rupa.
Ignorance and contact are the two main conditions that must be stopped. Craving conditions dukkha.
Most of the stuff in the dependent origination depends on contact, more than ignorance and craving.
You say that vedana cannot lead to tanha, by using some proof by contradiction and the concept of necessity. On the contrary. Vedana like you know is typically pleasant, non-pleasant and nor pleasant nor dis-pleasant [typically in 4th jhana]. SO that's the ideal basis for craving to pop up. Once there is a pleasant vedana, there is typically delight,and anybody wants more pleasant vedana.
"Brethren, if there were not this satisfaction which comes from
scents, beings would not lust after scents.
But inasmuch as there is satisfaction in scents, therefore beings lust
If misery, Brethren, pertained not to scents, beings would not be
repelled by scents.
But inasmuch as there is misery in scents, beings are repelled by
If there were no way of escape from scents, beings could not escape
But inasmuch as there is a way of escape from scents, beings do escape
the buddha even says how to see the 3 vedanas, in order to progress on the path
One who has seen the pleasant as painful And the painful as a dart,
Seen as impermanent the peaceful feeling Neither painful nor pleasant:
He is a bhikkhu who sees rightly, One who fully understands feelings.
But vedana is conditioned by contact, not by craving,
and once there is vedena, there is craving, and craving is the ''patipada[=path] to vedana''.
Just like the noble path is the cessation of dukkha, the opposite of noble path that is tanha is the path to contact, which is the condition for the arising of vedana [you do not need to do anything than craving more to get more vedana through contact]. craving as a path is really a super simple path: the path is just craving.
On the contrary, the opposite of craving-as-path has plenty of branches, from right discernment to the stilling of the Sankharas to their destruction.
Puthujjanas who love the idea that there can be perception without vedana are the rationalists. They love to call such a fantasy ''the objective perception'' and they always say that vedana pollutes the knowledge, the perception and the reality [puthujjanas who are rationalist say that the reality is the objects living in a world devoid of humans]. Those people say that the pinnacle of being a human is to be a free thinker, meaning a thinker who is not influenced by other humans, nor swayed by emotions, devoid of biases.
Of course, the dhamma has nothing to do, nor by goals nor methods, with the moronic fantasy of the rationalists.
If you want to know more, there is a whole vagga on khandas, on nidana, on Salayatana and there is a whole Samyutta on vedana
Typically this stuff is what the sutta in Saɱyutta Nikāya are about. Sadly, not many monks talk about them in a systematic manner. Bikkhu bodhi has done MN and is in the middle of AN right now, so it will be at least one year before he finishes AN, and perhaps he will do DN or Kn before SN.
Another famous bikkhu did an exposition on SN, but also MN, KN and DN, Bhante Dhammavuddho
you can get the records of his readings here