Hiya.
You can downgrade 9.9 easily, don't need a hardmod. There is a chance that you brick it if you are stupid, but if you use SafeSysUpdater that wont happen, unless you intentionally force the system off during the downgrade.
Since you have a New 3DS, rxTools only goes as high as 9.5 on EmuNAND, so ReiNAND would be my choice, since you can be on 10.5 EmuNAND with ReiNAND.
Use
this guide but get
SafeSysUpdater instead of the one used in the tutorial and you should be good. There are some steps to take care of after the downgrade, but at least the scary part is done when you've downgraded
If you have any questions whatsoever don't hesitate to ask! =)