How to define reasonable observed data

Questions
6

Do you mean sampling from the prior predictive? If so, you can do:

with pm.Model() as model:
    x1 = pm.Normal('x1', mu=0.5, sd=0.5)
    x2 = pm.Normal('x2', mu=0.5, sd=0.5)
    y = pm.Deterministic('y', x1 + x2)

with model:
    trace = pm.sample(1000)

and then look at the trace of y.