Tuning proposal in Metropolis

Questions
2

Yes, you can do the following:

In [6]: S = np.asarray([2.5]) # <-- custom proposal sd
   ...: with pm.Model():
   ...:     ... # your model
   ...:     step = pm.Metropolis(S=S, tune=False)
   ...:     trace = pm.sample(1000, tune=0, step=step)

Yes