Sealing tile grout

You didn't read the SDS and TDS then, before deciding that the product was suitable?

Yes.

How long is this piece of string?

I would:

Again it might but I would not risk it. (I don't normally even grout the internal corners, preferring to seal and get a better key for the sealant.

After 20 years the only problems I have had with unsealed BAL plain cement grout are slow etching by shower cleaner - a sort of mild acid.

You shouldnt need to do that anyway.

Almost all grouts are cement based. Epoxies are way more cost. Grouts slowly get filthy. Lithofin etc sealer helps keep it cleaner. Your choice. Cement grouts aren't flexible to any significant extent. The concept of flexible cement grouts is a game really. Whether you need flexible in a corner depends on whether there's any history of the wall cracking there.

Oh, don't lets get started on walls. This house not only does not have any flat or vertical walls, but a trip into the loft shows most of them are not secured up there at all, and thus do wobble a bit. Its been like that since 1939, so I doubt its dangerous, but avoid trying to sink a socket box into one side with something percussive or as mine did the ceramic tiles on the other side tend to fall off. Brian

What is behind the tiles? Plasterboard? Aquaboard(or similar)? Other?

Plaster board

My leaks happened because the backing wood swelled and split the grout Now the wood has been replaced with (as it happens, kingspan) something that doesn't swell when damp, the tiles are stable and the grout is a water barrier even though its not sealed.

Remember, this is not a swimming pool, Tiling gets splashed, once or twice a day. As long as the grout doesn't leak after ten minutes of showering, then it will dry out subsequently.

Like mortar and brick in a wall, it doesn't have to be water proof, just resistant enough to stand a good soaking without water running down the back side. And, if you have used a waterproof cement to hold it all in place, there isn't anywhere for the water to go anyway. Braces and belt is one thing., braces belt and a piece of string is simply OTT.

My experience both of my own issues and various DIY friends and former owners, is that the greatest single cause of bathroom leaks is flexibility and movement of the underlying structure.

A tiled plasterboard wall has very good rigidity. If it flexes enough to crack grout, sealing it wont help anyway. Ergo if your bathroon is flimsy, beef it up as a first measure.

Don't waste your time sealing grout.

Any small leak through cracks in the grouting is likely to swell or degrade the plasterboard (even water resistant plasterboard). In this type of installation I personally would silicone up and vertical wall joins as this is the most likely place for a crack.

It all depends on how well the tiles have been cemented. I've taken tiles off where the securing method has been more like dot and dab with little or no cement to the edge of the tile. When the grout cracks water can get straight to the underlying wall structure.

I have never seen a small crack in grout

I think that's because of your bed of granite approach to tiling and installing flexible things like acrylic baths.

No, the cement bed is under the shower tray :-)

But even a flexy bath wont crack grout. Just the bath seal.