Tub to shower conversion plumbing questions
This is way too much to answer in a single question, especially without pictures... but here goes:

...it looks like we can't re-use the same drain line for the shower as we have in the tub. Would it be better just to cut the drain line back to main drain line and run new shower drain?

You certainly can use the same drain line but it will need to be altered to exactly align with the drain assembly in your new &quot;stand up shower kit&quot;, which may necessitate cutting it back to reconfigure it.

...we don't need the overflow drain cover anymore... can we just cap that section?

The &quot;overflow drain cover&quot; is just a part of an entire bathtub drain unit called a waste and overflow assembly; which should be long gone since you state that &quot;the tub is out&quot;. All that should remain after bathtub removal is a trap and drain line (which will likely need to be removed/reconfigured for the shower, see above).

Is there an easy (no soldering) way to convert two faucet handles (hot &amp; cold) into one shower faucet handle?

NO there is nothing &quot;easy&quot; about plumbing when you are a &quot;complete amatuer&quot;; however, it may be a fun learning experience from which you will emerge more confident and competent.
I presume you mean to replace the old two-valve/two-handled tub filler valves with a modern single-handle/knob shower control valve. This might be accomplished with some new-fangled fittings that don't require soldering (I will assume copper since you mention solder) but I would recommend that you solder the connections (which means you get to buy a torch and a bunch of extra fittings and some pipe to practice with until you have a new skill that you are fluent with).
You must fit the new valve and drain to the shower kit to ensure that the pipes in the wall and floor are positioned perfectly, which means that you will probably need to install and uninstall (&quot;dry fit&quot;) the assembly multiple times. Follow instructions and have patience.