I assume you are going to use ASM, aren't you?

1- For the large DIM tables one tablespace for each one of them. All small DIM tables in a single generic tablespace.

2- Auto works just fine in our shop.

3- Since your business requirements call for running ETL during business hours you just can't drop your indexes(*)
What you can and should do is setup a process at the end of your ETL checking how large your bitmap indexes got, when they reach some threshold like twice the normal size just rebuild them, only the offending ones, much better if you can do this in a partition by partition basis.
As per rebuilding all of them during the night... depending on DW size and number of bitmap indexes you might find the night is not long enough to rebuild all of them.

(*) Just as a side note, if you are planing to resort to bitmap-join indexes you have to drop/create them

Just my two cents.