Yes. Luma3DS is recommended.
No, and there won’t be one. The elevated permissions required are complicated to support over multiple FW versions.
It only shows games for which you have the title key. And even then some games won’t show because they’re not cached server-side (this will eventually change).
All DLC is lumped together with the same title id, so even partial installs will show it as installed. If you’re missing DLC, just delete it and reinstall it. If eShop partially installed it, or you previously purchased some DLC through eShop on that console, you may lose the DLC freeShop installs when you revisit eShop. This is currently unavoidable.
Not yet. It’s currently unknown how to go into sleep mode without interrupting network services.
Depends on your CFW. Most of them allow it, including the recommended Luma3DS.
Even modding your console’s software/firmware runs legal risks. But like running freeShop, the risk is minimal. If you make sure to only install games you’ve purchased, then you remain in a safer legal grey area. And there is no way for Nintendo to know which client IP owns which games. That would requires some analytic guesswork, potentially resulting in many false-positive bans. Not likely to happen and none have been reported.
You likely have a FW version < 9.6 and therefore cannot play newer games that utilize crypto seeds. You can fix this with a FW update.
You’re probably not using correct encrypted keys (encTitleKeys.bin). Decrypted keys will give this error.
The system is reporting that you don’t have enough space on your SD card to install the game. If it’s a DSiWare game, then it’s your NAND that doesn’t have the space.
There are many possible reasons for this. But one known diagnosis is a possible SD card problem. For example, a fake SD card who is reporting space available while it’s actually full will often give this error. Double-check your SD card.
This happens with the installation state gets screwed up. You have to reboot the console to get rid of it. If you consistently get this, report how so the bug can be fixed.