The short answer is 'No', as you already found out through your research. A common property of most (pseudo-)random generators, including the one used by Minecraft terrain generator, is that 'similar' seeds give wildly different results.
There is no way of predicting that e.g. negative numbers will produce certain features more often. As for the letters in the seed - if the seed contains them, the string is converted to a numerical seed using a certain calculation (
String.hashCode() to be exact).
That said, there is some difference between numerical seeds and seeds with letters, but it has almost no practical implications: numerical seeds are 64 bit numbers, while seeds with letters are hashed to a 32 bit number. This means that there are 18 quintillion possible Minecraft worlds, but if you use a seed with a letter in it, you can generate 'only' 4 billion of them.