Fusion Drive - how to securely erase both parts? And, which tech solution is better?

I have an iMac 2013 21.5" whose hard drive was compromised and in deed of replacement. Currently, in Disk Utilities it showed:

• 121.33 GB Apple SSD ... and a 1 TB Apple HDD HtS5 .... separately.

Based on a little research it now looks like the Fusion Drive has separated. I secure erased using disk utility (highest level) the 1 TB Apple HDD HTS5 ... as I am planning on taking it in for service, but the 121.33 GB Apple SSD while I erased this - there were no "secure erase" options and I am concerned that there may still be business data on the SSD. Prior to this I used Filevault. In addition I am taking the iMac in for service but am unsure which option is best for this iMac (details below)

Three questions:

1. To be sure that all business related material is off the computer, how do I securely erase the 121.33 GB Apple SSD?
2. Have I securely erased the 1 TB Apple HDD HTS5 ... I chose the highest level and it ran for almost a day.
3. I have talked with two service technicians. One wants to put the fusion drive back togther with and 1 TB SSD replacing the old HD and the other wants to put in a 1 TB SSD and use that for everything and just leave the old 121.33 GB Apple SSD in there - he says the latter will be accessible.

I hope I have provided enough information and look forward to your suggestions and thoughts.