Basically u r trying to simulate Wilkinson div with out 100ohm,
Q1 ans: reflections are not happening to source as it is matched. so loss is ideal.
reflections are between 2 and 3 causing poor isolation.
Q2 ans: having 100 ohm between port 2 and 3 leads 25 ohms if 50 ohm load present at port 2 and 3.
this 100 ohm sees lambda/2 distance means potentials are opposite to each other and cause signal absorption at all times and improves the isolation between port 2 and 3.
hope this explanation makes you better understand.