AS 3600

1. For most cases it's an engineering decision. Personally I like to use a wall if the aspect ratio is >=4:1, especially where the length >= max mesh distance. Keep in mind, a column connects to the slab mesh at a single point, a wall connects at a line of nodes. For large columns or thick walls, some users add a thick block of concrete, centered vertically with the surrounding slab and the same area as the support to further stiffen the joint.
2. Correct, we only do punching on columns. Performing a punching check around walls, or groups of columns, adds another level of complexity that we have not yet been able to implement. You can still get wall reactions and check it manually if needed. 
3. See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/2059/ram-concept-modeling-and-meshing-faq#walls 
4. Many codes give recommended cracked section factors for analysis.  It won't make a lot of difference in the slab design except for thick walls fixed to the slab. 

Thank Seth for your explanation. It was really enlightening. I am trying to do an excel spreadsheet to manually check punching shear for wall or column with whatever dimensions. What confuses me is finding the value for σcp (average intensity of effective prestress in concrete, MPa) to apply into the formula here of AS3600-2009 section 9.2.3. Could you show me step by step to find this value in Ramconcept? Thank Seth once again.