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Rubbish in, rubbish out! No point using software if you don’t understand any of the principles behind it…

254 x 102 x 22 UB - there's your issue! Massively undersized for a 7.5m span...

I'm surprised that whoever has designed that has managed to get a 7.5m opening to work using a UB section. I'd expect a hollow section like an RHS would be needed due to the torsion from the outer leaf - probably why its started to rotate...

Hardcore and type 1 directly onto firm ground, sand blinding, DPM and then concrete. Sand blinding stops the membrane being pierced by sharp stones.

You need to redig the bottom of the trenches to get rid of all the slop and soft material once you've pumped the water out and before you pour any concrete

Stud wall possibly supports the rafters which allowed the original purlin to be removed when the loft conversion was done. I’d investigate properly before you do anything to the wall

Total length of the two window openings / 6

No the wall doesn't necessarily need to be at the same position as the original truss joint - whoever is designing the loft conversion should make an assessment of the rafter sizes and determine the required strengthening based on the position of the stud wall. As you push the wall further...

Prestressed lintels are precast lintels - the prestressed just refers to the reinforcement within the lintel.

A 3m high stud wall with no restraint at the head? It’s going to be wobbling all over the place surely?!..

Either joist hangers or just notch the top of the joists and sit them onto the bottom flange of the beam

Yes a sand blinding is recommended to stop the DPM being damaged - particularly over hardcore as any larger sharp stones can puncture the DPM if they are walked over. If the concrete floor is level and smooth and whoever puts the DPM down is careful not to damage it then the sand blinding isn't...

Yes you can drill holes in the joists. There’s a few rules about hole sizes and spacing, but for such small holes it’s not worth worrying about

Its concrete, just without the fine aggregate that you normally have in the mix. How thick is the existing slab and screed that you're taking out? You'll need minimum 100mm concrete for your new slab, so by the time you've excavated the 100mm level difference and then another 100-150mm for...

What about building regs? U-value for the floor?

Are you insulating below the slab?

Dig a hole and see what the ground is like! You'll need to excavate beyond any topsoil or shallow made ground to make sure you're founding on firm ground otherwise its likely to crack. A minimum of 150mm type 1 is usually standard for a ground bearing slab sub-base.

So you're digging 120mm into the ground to build the base of your slab. Will you even have dug beyond topsoil at that depth??

Not very easy to understand what you're trying to explain.. Whats the edge detail for your raft? Might be easier if you posted a sketch or some photos? For a raft i'd normally expect a DPM over the top of the raft (underneath insulation/screed) which then laps into the DPC?