>But look how often P shows up as a double letter, which stops you from rolling the keys because of the double-strike: opposition oppression stopping appendage application applying appreciate --- there's a lot.

Why isn't the location of a letter commonly struck twice only a problem when the finger used to strike the letter is also used for preceding and following letters? E.g. 'stopping' in Workman has right ring finger hit all of 'opp' which, yes, isn't great, but that's the only example I can think of - *opp* words.