So, perhaps, I should restate the issue at hand as I see it:

When should a GM declare that a unit's order is unadjudicable and therefore defaults to 'Unit Holds'?

Ok, so what would your answer to the ? be?

The manual takes a stab at answering:

Any vague or invalid orders are ignored.

Isn't that pretty clear? Any order is enacted as written, unless it's either vague or invalid, in which case it's ignored.

A Lon-Bel. Vague? No. Invalid? No. So it is enacted as written. If a fleet is in ENG and it orders a convoy, the order may succeed. If not, it'll fail.

A Lon-Mun. Vague? No. Invalid? No. Impossible perhaps, but yet a properly written move order from one province to another which will certainly fail.

A Lon-Neptune. Vague? No. Invalid? Yes. (have no idea where Neptune is on this map). So it is ignored. Unordered unit is considered ordered to Hold.

A Lon-Nor. Vague? Yes. Nor could mean Norway or Norwegian Sea, so it is deemed vague and is ignored. Unordered unit is considered ordered to Hold.

(A GM may consider that one not vague if he considers that an army cannot go to the Norwegian Sea - but vagueness is purely whether it is vague to the GM)

F MAO-Spa. Vague? Yes. Could mean either coast, so ignored, Holds.

F Mar-Spa. Vague? No. Can only go to one coast, so it is not vague. Ordered appropriately.

A Lon-Bol. Vague? Certainly. Bul? Hol? Bel? Any of those would be a slight typo from a valid space, but there is no way to know the intent. Unit Holds.

A Lon-Bra. Vague? Tough one. There is no Bra on the map, but there is Bre, and that seems to make sense, but this would come down to GM interpretation if he considered this vague or not. Personally I think this should be a misorder, as Brest doesn't even contain an 'a' there's no reason to assume this is Bre.