There have been a lot of previous threads on here, often under titles about insulation rather than specifically shed building.
Q1/ Gravel is for greenhouses. There must be damp proof course (DPC) to stop vapour rising. Even a typical garage concrete floor that doesn’t have have a DPC (most don’t) or an existing shed wood floor needs a plastic sheet followed by flooring eg chipboard at least.
Q2/ either lots of extra joists, or boarding or you can cut holes under bench legs and create a solid fill down to the ground. Beware compromising your DPC. Ideal would be 4in reinforced concrete over 4in of insulation with 4in insulated gap between floor slab and wall foundation to avoid a cold bridge. ( yes I know that is rather difficult)
Q3/ High level burglar discouraging windows save wall space for important stuff. Fit shutters for insulation, security, sun shading. Half the time you are in there it is dark outside anyway.
Q4/ If that space isn’t enough for you then you have bigger health problems to sort out before building a shed.
Q5/ Read the other threads on here and articles in ME. DO NOT follow any of that rubbish about ventilating the shed when you are not in it. Aim to make it totally sealed and run a dehumidifier, then enable controlled fresh air supply when you are in there.