"92-5/8"
"Q: Why is the length of a precut stud 92-5/8 in.? With the sole plate and two top plates at 1-1/2 in. each, the total wall height is 97-1/8 in.Sep 1, 2002"
https://www.finehomebuilding.com/2002/09/01/do-precut-studs-make-sense
You DON'T want a 8'-3' floor/ceiling height, you want a 8'-1' floor/ceiling height. You want to put 1/2" (or 5/8") sheetrock on the ceiling, then 2 sheets of 4' wide sheetrock on the walls. That gives you an allowance of 1/2" at the bottom. 8'-3" gives you an extra 2 and 1/2". Who wants to cut 2" slices of sheetrock to attach to the wall, who wants to tape and float the extra joint throughout the entire house?
Pouring a concrete slab in sections defeats the purpose of the monolithic slab. Each section will lift or sink individually and cause an uneven floor. You'll have construction/expansion joints here, there, and everywhere. This 'double wall' you're talking about, the existing wall and the 'new interior wall'. The 'new interior wall' will need a solid footing not broken up with 'joints'. Same would go for your load bearing walls, they would need at least an 8" thickened slab that would be continuos with #4 or #5 rebar.
"skid-steer", I guess that is a Bobcat with bucket attached? Yeah, you would have to find someplace that you could open up, especially if this is a raised house like you say it is.
You're basically gutting the entire inside of the house, how great does the outside look for you to try and salvage this piece of shit? Did you look in the attic? Whats that look like?
All Granny really needs is a little Studio Apartment type living space. Is this really for Granny, or for some resale value after Granny checks out? Or, ... are you just drumming up business for yourself?
I have no idea why precuts are that length. For a wall 8'-3" high just use two widths starting from the top, then a 3" width at the bottom that will not need to be finished well since it will be covered by baseboard.
Monolithic is a misnomer. All floor slabs have control joints, largest size without 12'x16', and rectangles should never be more than 1.5 times their width. On commercial jobs we use control and expansion joints. If a slab sinks relative to the next, the soil is unstable, which this is not the case here, again based on the stability of the perimeter footings.
The new perimeter wall will be over the footing overhang. Interior load bearing walls will be over strip footings.
As the OP states, "The brick veneer is perfect...".
The "attic" is a "sleeping attic", complexly finished.