Q1: You can try with *CONTACT_TIED_SURF_TO_SURF to tie the two parts, instead of merging nodes.
Q2: Contact will be required to transfer contact force between any parts that are not connected through the mesh.
Q3: In AUTOMATIC_SURFACE_TO_SURFACE contact the order of master and slave surfaces does not matter. In the SINGLE_SURFACE contact, all the parts coming into contact or self-contact are included in the slave side.
Q4: Yes, those are the most commonly used sliding contacts. You can start with setting SOFT=1, which is a node-to-segment contact. SOFT=2 will activate the (more detailed but also more costly) segment-to-segment contact algorithm.
Q5: AUTOMATIC contacts account for the actual shell thickness offset by default, therefore, the nodes of the shells can be placed along the mid-thickness of the shells.
Q6: To achieve quasi-static loading in explicit, you will need to apply the loading with a slow enough rate. You can judge about the contribution of the dynamic effects by looking at the kinetic energy in the GLSTAT output file. I common rule of thumb to characterize the behavior as quasi-static is that the kinetic energy is not more than 1% of the internal energy for most of the analysis time.