Anyone can ensure me I'll get the Aperture Labs Hard Hat if I buy Potato Sack Reunion?

I very much doubt it as they are not preorders

you will

According to a thread in the steam forums they took the hat out of the pack again, I didn't buy the pack so I can't confirm this, though.
Look HERE and following posts.

The hat was a leftover from last year sale and it was not meant to be in this year's bundle, at least everyone suspects that, but they were surprisingly sloppy, putting those games again for sale and not noticing it has last years promo still attached to it.

The bundle has ended... But Valve just send me a mail: Clicky