Essentially Yours Adjustable Fireplace Tray | Stainless Steel Expandable Ash and Ember Holder Pan | Nuts and Bolts Included for Additional Stability

Q: What is the depth (front to back) not length?

The tray is 15 inches front to back

Q: Can this be assembled to be square?

No the sides are not squared. They angle to the back of the fire box

Q: Is this safe to use as pan for charcoal grilling?

If you're using a grate to put the charcoal on and this is intended to catch the ashes, it shouldn't be a problem so yes. If you're planning on putting the charcoal directly in this pan and somehow rig up a grate above that for grilling your meat, I wouldn't do that. Even though this is stainless, the heat from the charcoal will take it's toll on it, but also the coals won't get sufficient air to burn efficiently. Charcoal grills are relatively cheap so use the right tool for the job.