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alternatively for 1)

=CHOOSECOLS(H3:L999, XMATCH(V7, H1:L1))

as for 2) I would recommend you create some LAMBDA functions for each equation A-E.  Here is an example for A:

FofT_A = LAMBDA(T, IFS( 
                    (T>0)*(T<=0.1), 0.8 + 15.5*T,  
                    T<=1.5,  IF( 0.704/T<=2.35, 0.704/T, 2.35),
                    T> 1.5, 1.056/T^2,
                    TRUE, 0 ))

Then I would use a CHOOSE and maybe a MAP over all the Time values:

=MAP(N3:N999, LAMBDA( T, CHOOSE( XMATCH(V8, H1:L1), FofT_A(T), FofT_B(T), FofT_C(T), FofT_D(T), FofT_E(T) ) ) )

I recommend you have those A-E functions separate so you can easily use them anywhere on the sheet but if you need a single formula it might be something like (but you need to fix the values for each of the sub-LAMBDAs:

=LET(TimeVals, N3:N999, DropDown, V8, 
FofT_A = LAMBDA(T, IFS( 
                    (T>0)*(T<=0.1), 0.8 + 15.5*T,  
                    T<=1.5,  IF( 0.704/T<=2.35, 0.704/T, 2.35),
                    T> 1.5, 1.056/T^2,
                    TRUE, 0 )),
FofT_B = LAMBDA(T, IFS( 
                    (T>0)*(T<=0.1), 0.8 + 15.5*T,  
                    T<=1.5,  IF( 0.704/T<=2.35, 0.704/T, 2.35),
                    T> 1.5, 1.056/T^2,
                    TRUE, 0 )),
FofT_C = LAMBDA(T, IFS( 
                    (T>0)*(T<=0.1), 0.8 + 15.5*T,  
                    T<=1.5,  IF( 0.704/T<=2.35, 0.704/T, 2.35),
                    T> 1.5, 1.056/T^2,
                    TRUE, 0 )),
FofT_D = LAMBDA(T, IFS( 
                    (T>0)*(T<=0.1), 0.8 + 15.5*T,  
                    T<=1.5,  IF( 0.704/T<=2.35, 0.704/T, 2.35),
                    T> 1.5, 1.056/T^2,
                    TRUE, 0 )),
FofT_E = LAMBDA(T, IFS( 
                    (T>0)*(T<=0.1), 0.8 + 15.5*T,  
                    T<=1.5,  IF( 0.704/T<=2.35, 0.704/T, 2.35),
                    T> 1.5, 1.056/T^2,
                    TRUE, 0 )),
MAP(TimeVals, LAMBDA(T, CHOOSE( XMATCH(DropDown, H1:L1), FofT_A(T), FofT_B(T), FofT_C(T), FofT_D(T), FofT_E(T) ) ) )
)