What are the rules for pluralising abbreviations?

Especially in academic writing, pluralisation of abbreviations is a very handy device: ‘p’→‘pp’¹ is ‘page’→‘pages’, ‘ed’→‘edd’ becomes ‘editors’, ‘ll’ is ‘lines’, ‘nn’ is ‘notes’.² The rule (given ‘ed’→‘edd’) appears to be that the final letter of the abbreviation is doubled in order to pluralise it, but is this correct? So, as the question states: What are the rules for pluralising abbreviations? From this follows two sub-question: Are there any notable differences between academic writing and writing for a general audience (except perhaps avoiding too many such abbreviations)? And when using established Latin phrases in English writing, are there any special rules to consider for pluralisation of Latin abbreviations in English texts?

With reference to note 2 below, I am conscious that these abbreviations might have been inherited from mediaeval scribal practice. Could the answer perhaps be found there?³

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1.   Or ‘pp.’, henceforth without the full stop.

2.   Or ‘pāginæ’, ‘ēditorēs’, ‘līneæ’, ‘notæ’.

3.   Could perhaps ‘qq.v.’ be a legitimate variant of ‘q.v.’, ɔ: for ‘quibus vidē’ rather than ‘quō vidē’? An example (if correct) could be ‘These issues have been discussed earlier, qq.v. nn. 13, 27 and 29.’

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Comment 1

I considered adding the latin tag, but chose not to, as this question isn’t about Latin specifically, but rather what is customary in English. Should you consider that the latin tag indeed belongs to this question, do please say so in the comments, and I will edit it.

Comment 2

As pointed out in the comments, we are here dealing with abbreviations renegated to footnotes. I still believe the question holds relevance, though, but it is worth keeping in mind.