Issue 1: I bought the STIs before I decided on the final drive choice, so I wound up with a used set of 3 x 8 in good condition for $50. If I am going to have indexed front shifting, would it be adviseable to replace my current 52/42 chainrings with ones that are ramped and pinned? Would front index shifting work with unpined/unramped rings?
The current chain rings will work fine. You will however notice a slight delay when upshifting to the larger cog. Actually, you won't notice any delay because you have been using those same chain rings, but if you are used to grooved rings, that is when you will notice the slight delay.
My main concern was having a low enough gear for riding Southern Indiana hills, and the original setup of 42 front ring and 28 rear cog as my lowest gear was still a little too tall for my taste. I originally was going to have triple front setup by adding a 30 tooth granny ring to the Sugino crank, but decided to go compact, and can get a 50/34 set of rings and just run dual.
Sounds good to me.
You probably already know that the ratio is 3:1, so the best way to achieve an "easy" gear ratio is in th rear, but the 50/34 will give you a pretend 27 cog in the back on the 50, and a pretend quasi-28 on the 34, this is if we are comparing to the 42/53.
Issue 2: I want to confirm, but I'm pretty sure that I can use my 8 speed shifter with 7 speed cog, and three speed front shifter with 2 front rings.
Yes, you can. The cable pull/throw ratios are minutely different, but you can surely get away with it. If the derailleur is tuned in right, you will not notice the discrepancy at all.