Hi Otto: For 1) You're right, but there is no secret. As you said, it is because the "failure point" for plastics is almost never given as a yield stress. I have read some plastics manufacturers use a value such as "strain-to-failure": how far it stretches before they consider it failed; or "fracture modulus": how much strain energy it can take before they consider it failed; or, how far it deflects could be a failure criterion. I'm sure there are other measures, and they depend on each manufacturer's interpretation of just what THEY consider failed. The important thing is not to "stress" about it (no pun intended). The Yield strength in SW Simulation is only used to calculate an FOS. That said, just put in a number - it won't show up anywhere unless you run a FOS calculation. Refer to the manufacturer's information about what measure they use to consider "failure".
For 2). If you can stick with the linear, elastic assumption (not a bad assumption for ball-park determination of performance), and an isotropic formulation (assuming the plastic does not have directional properties), you can get away with using the isotropic constitutive relations. I hear it is a lucrative profession to conduct testing on various manufacturer's materials and sell them the test information so they can actually publish the values for strength-x, shear strength xy, compressive strength x and y, etc. It is expensive, and because of that, many manufacturer's just don't do it. Just stick with the material values for elasticity (don't worry about the strength numbers since you don't have them and probably won't be able to get them without testing yourself). That is EX=EY=EZ, GXY=GYZ=GXZ = E/(2(1+Poisson's ratio)), for isotropic materials with the linear elastic assumption. It might be worth a call to the scientists at the plastics manufacturer and ask them point blank if they have a failure modulus or 'strain-to-failure' value you can use to predict strength. You can directly plot the strain energy in SW Simulation, as well as the strain energy density, and of course the strain. Hope that helps. Tony