It's not made very clear, but this sounds viable. There is a question in the FAQ, that asks if Ld is used for instant death and the answer was yes, so I think that it's safe to assume, that all wounding mechanics work as normal, you just use the models Ld value, instead of their T value.
Putting an upgrade or independent character into a unit doesn't increase their Leadership, it is just so, that for Leadership tests (this includes morale tests) you always use the highest Ld-value, if there are models with different values.
As a to-wound roll is obviously not a Leadership test, you don't use this rule, but the one used for to-wound rolls, that says, that you always take the majority (or higher, if they are tied), if there are models with different values.
I must rule with eye and claw — as the hawk among lesser birds.