Viagra does not cause a spontaneous erection. You have to be sexually stimulated in order for it to take effect. This particular film scene would suggest that Franco was sexually aroused by the 'chase'.
Hope this helps clear things up for you.
I will answer your questions in turn
How does venous leak develop?
WebMD in the linked article in your question states how Venous Leak can happen
It may happen with vascular disease.
Vascular diseases affect blood vessels. They lower blood flow to organs such as the heart, brain, and kidneys. If they cut blood flow to the penis, they can cause erectile ...
We can't give medical advice here. But I can guide you in how to approach talking with your doctors, and add another doctor/specialist you haven't mentioned but should see.
You have already told us that you have elevated prolactin - which
has a known association with erectile dysfunction.
You have already told us that you have depression, which has a known
Problems with erections can be caused by a physical condition, and they can be caused by psychological factors. Treating any physical or psychological underlying condition should help. Your GP can suggest possible treatment options.
Treating erection problems caused by psychological factors can be more challenging. However, most men who persevere with ...
I generally agree with jj78, although I would add a couple of provisos.
a) The answer partly depends on how one defines 'spontaneous', an adjective with more than one meaning:
spontaneous adj. 1. occurring, produced, or performed through natural processes without external influence: spontaneous movement. 2. arising from an unforced personal impulse; ...