I always found it peculiar that the author of the From-Hell-Letter used the phrase "Catch me WHEN you can". Isn't the right english Version "Catch me IF you can"? Or are there any varieties? I'm asking, because I am german myself and may be mistaken.
You have to know that "if" is translated into the word "wenn" in German, which is pronounced like the english word "when". So it would make sense to me, that this mistake could be easily made by a german-speaking author. What do you think?
I think an argument against your point, is that the writer actually says just before the "catch me when you can," "if you only wate a whil longer" - so he would be making a mistake just after he failed to make a mistake.
You're right "Catch me WHEN you can" ist a grammatical construction, but ist it really a common expression? Even in connection with the use of "if you only wate a whil longer"? In Germany we only know the expression "Fang mich, WENN du kannst!". Maybe I'm a little stubborn but for me it would fit just too well: The many wrong spellings, the use of the word "when" instead of "if" (which is a very common mistake among Germans who are not used to speak English) and many witnesses described him as "foreign looking"...
I am conducting research in to this case as I have done for years. The only German suspect I have come across is one Carl Fagenbauhn (apologies for the spelling). He was a German Midshipman who was making frequent trips between Germany, the dutch isles and London. Is he in your highly valued opinion a credible suspect. If not why not?
Getting back to the Lusk Letter, I've always wondered why he addresses Lusk as "Mishter". It's a wrong spelling, obviously, but not even a phonetic one. Does anyone know if Lusk had a speech impediment which the writer might have been mocking?
On balance, I don't think there's much in the letter to suggest that the writer was German, although Chriswald is entirely correct in saying that 'if' in English translates as 'wenn' in German. I think the 'Catch me when you can' reference is just an idiosyncratic variation of the usual wording. Alternatively the writer is implying that Lusk has missed (or will miss) an opportunity to apprehend him.