A1: The slave Deco unit will not be router mode, if you didn't connect the slave deco to main deco by ethernet cable, it would be like extender, then if you connect an ethernet cable between them, the slave deco would be working with ethernet backhaul, no need to switch the mode manually;
A2: Similar to the question 1, after you install all the 3 Deco normally, the main Deco is on router mode, the other two slave Deco are in extender or access point mode, when you switch the mode on the Deco App, it is changing the mode on the main Deco.