1. Size limit shouldn't break sequence steps. There might be something missing or inconsistent in the PVZ file. For the size limit, you can make use of the 'Cad optimizer' while uploading the resource.
2. Recognition of ThingMark depends on the extent of the curve. I would recommend using a plain surface
5. Loading time dependents both on network speed and the 3D model. Did you try cad optimizer?
For other questions, can you please post your questions at ThingWorx Studio Community so that we have a scope for broader audience.