Index FFS will scan all your blocks up to the HWM. Coalesce wouldn't help - it could help to relatively wide range scnas but not to FFS. You need to rebuild your index to get HWM down.
However, your longterm solution should be going into direction of questioning this statement - this is, probably, uselss work and doing something useless is alway sub-optimal versus not doing it.
Any suggestions as to what i should be looking further. I really don't want to rebuild the index :)
-- Best regards, Alex Gorbachev