1. If anything, this public dataset will increase 'collusion' by giving the parties in the market access to the integral set of market prices.
2. Collusion is a strong word. It hints at breaking the law, willfull action, etc. Looking at the price of a competitor and acting upon that, and internal information, is not collusion. It is completely legal and economically rational behaviour. There are hints and information everywhere on which to act. Groceries are an oligopoly, and they are ogipolistic buyers as well. Lots of information for grocers in the price bids of their suppliers, that all groceries get in tandem.
3. I've worked as an economist at one of those government agencies. They are filled with keen PhD / MSc economists looking to bust kartels and find collusion. Geographical price dispersion models are so '00s. There is a lot of positive energy and interaction between academia and government. Actually _proving_ collusion is terribly hard. Prices that move in parallel is not collusion. Proving collusion is finding documents, emails, phone calls, tracking cars with staff members of various firms to motels , paper trails of inside information in another firm. That's collusion. And I fear that's not what happens in the grocery sector.
The motel one comes to mind as real life example of how DoJ in the US prosecuted a proper cartel. The talk was by more a broad-shouldered officer type, than an economist. Certain set of transport firms always had a interesting way of responding to auctions. Always the second best offer was quite far behind and there was a pattern to which firms did or did not reply to a bid request. End of the investigation, they could trace employees of the firms to meeting in a motel before bidding. And somebody left a paper trail.