There's nothing in Harry Potter-style time travel (either canon or MOR!verse) about not touching or interacting with past versions of yourself.
I must have missed this. Where is it written that you can touch your past self, mingle magics with your past self, cast spells in your past self, etc.?
Non sequitur
Privileging the hypothesis - that mingling with your past self is harmless. It's the rare timetravel fiction (fanfiction or otherwise) where such interactions are harmless; usually, it's disastrous in some respect. In the absence of an actual example that it is not disastrous, like the Tom Riddle citation, our priors are not 50/50 or outright assuming it's harmless.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: