Postctere a parasol in a tub

Yes. Put cold water into a bucket. Add dry postcrete to the water to mater a runny slurry like thin custard. Pour the liquid concrete into the "mould" (tub or bucket or whatever). Don't worry if you think "this is too wet, it will never set - I've put too much water in". You haven't. Insert the spike or push a suitable sized diameter longish pole (1m) into the wet concrete mix. Postcrete goes off very quick (20 mins) so do it quickly. Whilst it is still wet check the pole for upright and make sure it is where you want it. Let the postcrete go off for 20 mins then take the pole out carefully. It will be useable in 12 hrs.

Depends on the tub. You can buy a builder's rubber disposal bucket. They can take some punishment, the don't crack like plastic buckets do. These should also be able to take the heat as postcrete is exothermic in reaction. Or..... You could buy a proper stand for it. If you postcrete it in that's it! You aint moving the umbrella!! You cannot use it with a table, if you do then the umbrella is permanently in the table. What if the umbrella breaks? Its stuck in a bucket of cement and the WHOLE LOT must be thrown away.... I hope the descriptions steer you to just getting a solid base? Cheap isn't always the best option, the postcrete will be very heavy to move.

Yes...maybe cheaper to by a base...

Watch for the point of the parasol piercing the tub and allow for a suitable tub as postcreate give off a amount of heat. Check the cost of a parasol base before making your own

Yes you can theres no reason why you cant

Yes you can, but make sure that the tub is big enough for the size of parasol.

Yes use post create