Sorry but Tyler Durden is talking pure imaginative theory with no practical substance. Install drainage, which drains to where exactly. Install gravel and sand and put walls on it? Gravel, the roll around stuff? And sand the soft stuff?
Still sound like something you want under your walls? No.
Sure dry stone walling stands the test of time, provided there are zero, and I mean zero side thrust loads applied to the wall, ever. And definitely means no soil.
Mortar helps the wall absorb loads by simultaneously sticking the units together AND holding them apart. Probably the reason the mortar didn't stick (assuming you have proper/suitable mortar) is that your slabs were dusty/dry and the mortar was probably too dry.
To cure: Wash all dust off the slabs with a hosepipe and allow the surface water to soak in (the slab will still look dark/wet). Mix the mortar using enough water to make a smooth, creamy mix that doesn't crumble.
Lay slabs using the nice and creamy mortar and tap into place, remove excess that squeezes out with a trowel and once the mortar has hardened off a little, tool it off with whatever finish you want (half round, struck with a trowel etc). Joint thickness shouldn't be too thin, aim for 10mm, give or take a few mm's either way.
At the end of the days work, cover down with fleece (or old blankets) and a layer of polythene. Keep 'green' mortar covered thus for a week or so.
At least thats how we lay brick/block/concrete stuff in the UK/EU...
Bed it in plaster (which is very likely what the tub manufacturer recommends - most do.) Foam can work but is not preferred (easier when you have an already installed tub, though.)
Bedding on plaster is literally get ready to go, test fit, dump a pile of plaster on the floor under the middle of where the tub goes, set the tub. Tub ends up solid, you don't have anything that's going to rot in there, and the plaster is easily broken out when the tub eventually needs to be replaced.
As for your cockamaimie scheme, the diving weights alone will cost more than the bag of plaster; and it's an ugly mess and waste of effort .vs. doing it right with plaster.
Best Answer
Contact of the bottom of a steel tub with a mortar bed could cause corrosion of the tub from the bottom. If the instructions do not call for a mortar bed, I would not use it without expert opinion to the effect that it is a benefit. A good quality steel tub does not need the support of a mortar bed.