Hi guys, just an update.

The tiles with the sealant are holding well. Di naman masyado madulas yung area pag umuulan.
My theory on the hairline cracks was that these tiles were "seconds" meaning mga sobrang pieces na mga ginawang batches. They are actually cheaper than the full price, around half the price. But if you intend to get these seconds, inspect them as much as you can.
Also, I should have used tile adhesive rather than cement.
But it's all good. happy pa rin ako. The sealant is holding and retains the shine of the materials.
Sulit!