coalesce first non-null, varying null positions in hierarchical table sql

I have a table with varying depths of nulll values :

Does anyone know how I can write a select statement to reverse & collapse the cols by NULL values? Take the first rightmost value as the new leftmost value, achieving this transformation :

You can concatenate all columns with some delimiter, substituting NULLs with some constant like 'null', extract null part, reverse not null part, concatenate again reversed not null with nulls, split to get an array, and now you can get arrays elements as columns, do not forget to check for nulls and reverse each element to get values longer than 1 character correctly:

with 
test_data as (
select 'v' as dim, 'c' as col1, 'b' as col2,    'a' as col3,    NULL as col4 union all
select 'x', 'a', NULL,  NULL ,  NULL  union all
select 'y', 'b', 'a',   NULL ,  NULL  union all
select 'z', 'd', 'c',   'b',    'a'
)


select dim,
       case when reversed_array[0] = 'null' then null else reverse(reversed_array[0]) end as col1,
       case when reversed_array[1] = 'null' then null else reverse(reversed_array[1]) end as col2,
       case when reversed_array[2] = 'null' then null else reverse(reversed_array[2]) end as col3,
       case when reversed_array[3] = 'null' then null else reverse(reversed_array[3]) end as col4     
from
(
select dim,
split(
concat(
--all before |null
reverse(regexp_replace(concat_ws('|',nvl(col1,'null'), nvl(col2,'null'), nvl(col3,'null'), nvl(col4,'null')),'(\\|null)*','')),
--NULLs
regexp_extract(concat_ws('|',nvl(col1,'null'), nvl(col2,'null'), nvl(col3,'null'), nvl(col4,'null')),'((\\|null)*)$',1)
)
,'\\|') as reversed_array
from test_data
) s

Result:

dim     col1    col2    col3    col4    
v       a       b       c       NULL
x       a       NULL    NULL    NULL
y       a       b       NULL    NULL
z       a       b       c       d