Cast iron pan flaking

So, just answering the new part of the question:

You can simply season "over the top" of the existing seasoning. To do this, you'd go through the normal seasoning process, wiping the pan with oil, heating in a 500F oven, wiping again, heating again. This will fill in the "holes" in your seasoning and you'll be able to use the pan again.

However, you will also end up with somewhat rough, uneven seasoning on the bottom. This will make things more likely to stick than if you completely reseasoned the pan. You can improve this by scrubbing the bottom of the pan with a mixture of course salt and oil. This will "sand down" the seasoning without completely removing it.

You should also ask yourself if maybe the original seasoning of the pan wasn't faulty, given that some of it already scrubbed off.

I own a similar square grill pan and have always had luck cleaning it with a stainless steel scouring sponge and lightly soapy water. The spongy texture is very effective at getting down into the grooves. No need to be especially vigorous - a couple quick passes will take out accumulated residues, and you can use a regular sponge afterward if you like. I've no idea how they season these, but it seems to hold up quite well over time. As near as I can tell, you can also re-season these just like any other cast iron when it does degrade.

Their statements about enamel are probably referring to the outer, colored finish - I also own a Le Creuset dutch oven which has the same enameling inside and out. Care and feeding of that surface is of course totally different from cast iron.