Actually, I already done what you said.
I bought slots with diamonds while playing for 4 days,
then I activated free slot to unlock the last slot row for backpack as well as 3rd slot row for storage chest
Above did not trigger a ban and after playing for nine days, I unlocked the rest of storage chest with normal diamonds
then I tried unlock 2nd storge chest with 2 rows of free slots and after >6 hours later, I finally received the ban notice.
It is possible that they reference VIP level with the number of slots unlocked
as to unlock those slots will use up diamonds which in turn will raise the VIP levels,
I was VIP level 3 when I unlock the free slots the 2nd time
which might be impossible as spending that amount of diamonds for slots should raise me to VIP level 4 or 5?