Answering your two questions (How is it connected to the rubber sheet touching it? Is it Bonded Contact?):
All contacts between aluminum sheeting, rubber and steel plate with the soil is bonded contact.
I performed the analysis, configuring the Automatic Time Step as you mentioned, however I get the same alerts.
I will try to do what you say in your message to better understand the M-C model. But as I am new to the program, I may have some difficulties.