a.) If you are going to use your model in different client also then you have to use 'Cross client' as if you are transporting it to some other client and choose it as session client only, it will show as blank in data preview.
b.) Can you please explain your second point with screenshot? didnt get you.
c.) Suppose you have given default schema as blank, in that case if you are using any table say BSEG, in that case you have to mention everywhere like "Schema_name.BSEG" and if you have given some schema name say 'ABC' in your default schema, then HANA tries to find your table in ABC schema only and you dont need to mention explicitly your schema name with your table like you can use your table name directly as BSEG where as if BSEG is not in ABC schema , in that case it will search BSEG in SYSTEM schema.
And it will be good if you keep your tables in same default schema and transport it to other clients.
BR
Sumeet