I'll try to answer to my best.

As for the immutable part, I'd say you're right but when I do some changes, they are inplace.

1/ con.insert(yourtable, {"col1": ['value1'], "col2": ['value2']}) | See https://ibis-project.org/backends/duckdb#ibis.backends.duckdb.Backend.insert

2/ con.raw_sql("DELETE FROM mytable WHERE col == 'value'") | See : https://ibis-project.org/how-to/extending/sql#backend.raw_sql

3/ I'd go for mutate | See : https://ibis-project.org/reference/expression-tables.html#ibis.expr.types.relations.Table.mutate