to answer the two questions:
a) It's intentional. I'd be extremely surprised if they changed it at this point (and I like it this way).
Yes, it is possible:
The original buildings can be found in the ImprovementDefs.xml. You can leave those alone, but rather create clones of the buildings in a separate file.
Let's say there are 6 factory tiers. Then the current upgrade path is like this:
tier1 > tier2 > tier3 > tier4 > tier5 > tier6
So far, so obvious, now what you need to add are these paths:
tierA2 > tierA3 > tierA4 > tierA5 > tierA6
tierB3 > tierB4 > tierB5 > tierB6
tierC4 > tierC5 > tierC6
tierD5 > tierD6
Each variant of the base buildings is named exactly the same, they are all copies of the base version, only that you remove the <UpgradesFrom> line from the first building of each variant. If you simply remove upgrading from the baseline, this will allow you to build the final version straight, but this will break upgrading buildings, which makes this effort necessary.
On the plus side, you'll be able to choose whichever version you want to start with.
On the minus side ... well, this will effectively multiply the items in the construction list (as the base version and all 5 variants will show).