Use a utility mod like MiniHUD or Litematica for building, but these don't exist for Eaglercraft. If you want to test mechanics, download the official Minecraft Java Edition and use mods like Carpet Mod in single-player.
A: "Safe" is relative. Some open-source clients (on GitHub with clear code) may not contain malware, but they still get you banned. Never run an obfuscated client from a stranger. 1.8 Hacked Client Eaglercraft
A: Unlikely, because Eaglercraft does not communicate with Mojang's authentication servers. However, your Eaglercraft username can be banned from individual servers. Use a utility mod like MiniHUD or Litematica