It works on o3DS as well, yes. arm9loader is part of the n3DS firm, but since the bootroms are identical on both models, an o3DS will load the n3DS firm just fine.1 a) I am super confused about whether A9LH is possible to install on O3DS. From what I understand, ARM9Loader is an additional security layer that is used only on N3DS, but OTP dumping guides and A9LH install guides also mentions O3DS...
If it's got the exploitable version of the browser, you can use 2xrsa for that. Full firmware version should be 2.1.0-4. If it's 2.1.0-0 or -2, you'll need to use Cubic Ninja.1 b) If it's possible to install A9LH, can I simply dump the OTP following this guide (beginning at Section III), and then compile A9LH using this tool? Is that all (aside from installing, I know I need to update to a newer FW to launch the HBL)?
Yes, same as previous answer.2) Can I make a NAND backup using the Decrypt9 from Section IV of the OTP guide? I mean, I don't know if it's a special build or something like this, and I really don't want to brick my console...