Nobody from eBay will be reading your post here, it's a member to member board.
I can address your concerns but first:
I would ask eBay to reconsider reporting, filing and remitting the sales tax on behalf of businesses registered with the CRA and allow us to continue doing what we have been doing so far.
Asking eBay is completely pointless, the new rules are set by the Federal Government, you don't seriously believe that this is something eBay thought up. The same rules will apply to every digital Marketplace that operates in Canada.
Presuming your are currently registered to collect GST/HST virtually NOTHING changes for you, Your filing dates remain the same, you would report gross revenues as you always have, claim ITC's as you always have and instead of sometimes having to remit taxes collected when they exceed the amount of ITC's at you claim you will now almost always be getting a refund since your GST/HST collected will be zero (for eBay sales).
As far as when eBay will be filing, although not relevant to you I suspect that due to the large amounts involved eBay will be filing on a monthly basis.
The only numbers you need from eBay are total sales and total GST/HST paid on fees (ITC's). Both of these numbers are available from eBay currently with 1 - 3 days of the end of the month via a Transaction Report and will continue to be available after July 1st. If you do need to know the amount of GST/HST collected by eBay that information is also included in the current Transaction Report.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.