Sorry, but I don't think it is a good idea to put up a standard letter. That is just seen as a petition by authorities and like it or not carries little extra weight. Such is the democracy we live in.
That's correct. Public representations will be summarised by officers and presented to councillors or whoever makes the decision; individual distinct comments will make it through separately whereas if officers get 487 copies of a form letter they'll just say "and there were 487 copies of this form letter".
However, I've just been reading through (for today's meeting of Cambridge City Council's environment scrutiny committee) a set of public consultation responses to a number of planning issues, and in one case it's quite clear where there has been a letter campaign in one village which has carefully managed to word its comments sufficiently differently that they get reported on separately to members. It's a pity that they're all complaining about something (a) that is outside the remit of what's being consulted on, and (b) isn't going to happen anyway - whilst you might expect the odd individual to make both these mistakes in the same letter it's a bit obvious that it's a campaign when dozens of people make the same two mistakes!
So, avoid form letters, and especially avoid giving people wrong briefings as to what to put in their letters!