I'd check it as is. You have a nice compression block - no deducts for the metal deck flutes. Bump your concrete up to 6000 psi for this piece and see where it puts you.
Many, many slabs-on-grade (including and especially pavements) are designed and cast completely unreinforced. The load goes through the slab into the base material and imparts minimal flexure to the slab; therefore, no reinforcement is needed.
No, I don't think it's appropriate to use the Chapter 13 component forces instead of the Chapter 15 non-building forces for design of the frame.
Logic would say that the anchorage forces from Chapter 13 would usually be higher than the frame-as-a-whole forces from Chapter 15.
Do you have...
If the building were designed without sprinklers 100 years ago, but the same building would require them now, I seriously doubt that an AHJ or Fire Marshall would let you omit sprinklers if you were to add a 4th floor. They'd make you sprinkler the whole building.
Looking at the beam size (W27x178) and all those 7/8" bolts, it seems like someone is hoping that stiffener somehow helps in shear. The column can't carry much moment, inadvertent or not.
This dovetails with our previous discussions about whether or not the column need be designed for the load...
We used to ask that the bottom of columns be "milled" to ensure full bearing. I believe that AISC many moons ago said that cutting the column "square" is adequate for bearing and small deformations from imperfections do not affect performance. You case may be similar.
You have received some great advice in this thread. Eng-tips is a great resource. Now my two cents.
First - be brutally honest with yourself. Are you really doing twice as much as the person next to you? Or are you taking 50 hours to do what should be done in 40? I don't get paid overtime and...
Have a look at the Vulcraft website. They have tools for diaphragm design. You can try various arrangements and compare to get equivalent capacities out of different fasteners.