Hi Ed.
Welcome to our Mattress Forum. 
Thank you for sharing your stats and shortlisted configurations.
Children and adults do tend to have different needs from a mattress as children are still growing. There are some general firmness guidelines for children in post #2 here, if you’re interested.
Like you’ve already mentioned, the separate 3" layers give you more wiggle room in terms of layer re-configuration. Also, should you find that 10 years down the line the top 3" needs to be replaced, you could very likely do so without having to replace the bottom 3".
No, no worries there.
Most side sleepers will fall in the range of from 2" - 4" - generally, 3" is an ideal starting point and then depending on weight, body shape, preferences, and the firmness of the support layers, to go up or down from there. A good starting point for a back sleeper is 2" of softer material on top of your mattress and then increasing or decreasing from there depending on other factors like weight, preferences, or other sleeping positions.
That’s hard to know for sure since neither you nor I can feel what she feels while sleeping on a mattress. It’s possible 3" is fine. It’s also possible 3" is too much. Another potential configuration you could consider is:
3" base (same firmness)
1" transition (firm for you, med for her)
2" comfort (med for you, soft for her)
1" can be quite difficult to find, though, at least in terms of latex. So, you may also wish to consider a 2" transition layer.
We have a number of experienced DIYers in the forum who frequently give great advice. I hope one (or all!) of them come across your thread here as well and chime in with their thoughts.
NikkiTMU