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Crusty old HVAC guy here from the dark ages.
1. Combining upstairs/downstairs is no bueno, it will always be hotter upstairs and you will never balance the temps.
2. I did a few big installs in Rolling hills with two and even one with three split systems and NEVER combined returns. Rule of thumb is you want the return is the same area as the supply.
3. Low air flow to add on's is typical, believe it us crusty, stinky, beer guzzling A/C guys actually calculate the size of every room and design the duct work (and air handler) to deliver a specific CFM for the square footage. So when you add on a room and extend the duct work to put in another grill guess what?
A lot of options but I don't have time to study your post (at work now) and think about your situation.
On some installs we would drop a package unit next to the house and build a metal plenum/duct work to connect to the duct under the house
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Gary Fisher 29er
2019 Kia Stinger 2.0t gone 
1995 Miata Sold
1984 944 Sold
I am not lost for I know where I am, however where I am is lost. - Winnie the poo.
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