Chipboard is a composite material and thus can delaminate. This means that it cannot be considered as a stable substrate. Any porous substrates should be primed with an acrylic primer which will penetrate the substrate surface and form a secure bond. PVA is basically wood glue and even when watered down will only form a skin over a substrate so you will only ever be fixing tiles to a wood glue skin and not securely bonding to the substrate.
As for your issues, I would remove the tiles whichever way you can as the dispersion adhesive is unlikely to be dry in the middle of the tile, so once you have broken the bond around the edges, they should come up with little resistance. You would be best to overboard the floor with 6mm cement boards which are fixed with a flexible cement based adhesive and then screwed at 150 to 200mm centres with a 4 x 20mm screw (that way you can be sure that you will not pierce a pipe).
I'm assuming that your tiles are porcelain if they are 600 x 600 so leaving them stuck is not an option as the adhesive you have used is not designed to bond porcelain (it must be flexible).
Sorry, but it is a case of rip it up and start again. And remember, the staff in a DIY shop are trained to sell products, they rarely have construction experience so their advice will only ever be basic.
Sorry,
Daz