I had an argument with a friend. He believed that R&D's secret lair prevents you from tapping basics as they have no text. However he also believed that he could cast his player reward Remove Soul to counter my Llanowar Elves. Who is right?

Basic lands being able to tap for color is baked into their rules and definition. R&D Secret Lair doesn’t stop that.

Shouldn't sip of Hemlock give a poison counter instead of life loss? : )

Shouldn’t Remove Soul fail to counter Soulless One? : ) Better gameplay means that flavor isn’t always 100%.

If I cast a Soul Summons and the top card of my library turns out to be a Soulless One, what happens?

The same thing that happens when you Remove Soul it - game mechanics trump flavor. : )

Why can't Swords to Plowshares deal with swords? That seems like a flavor fail.

And Remove Soul can counter Soulless One. Some flavor falls through the cracks for the sake of good gameplay.

Actually, Doug Beyer explained in his article "A Fresh Coat of Magic Paint" that Remove Soul was changed to Essence Scatter because ripping out a creature's soul sounds like a black kill spell, not a blue counter.

That too. : )

Are we ever going to get Terror back? I understand that Doom Blade is strictly better, but I really like when I'm able to play alpha/beta cards in standard that aren't basics or just unsummon. I'm also petitioning for the return of Remove Soul.

Terror has a non-regenerating clause that we don’t tend to use much anymore. Also, the nonartifact clause isn’t used much either.

If card names are one of the thinnest resources available, why do cards that are the exact same get printed with different names? I know sometimes it's flavor (Kodama's reach vs cultivate) but what about Remove Soul vs Essence Scatter, expanse vs wilds, etc?

Usually a name change is based on the old name not making sense in the new environment. Other times, it’s development wanting players to have access to eight copies in older formats. Also, the resource is not names but simple, elegant names.

How can Tragic Slip be used on a creature that flies? How is it supposed to slip in midair?

Here’s one of the things about names. Most names don’t make sense in every situation. For example, is a kill spell supposed to always make perfect sense depending on size and abilities and color and creature type. Yes, it’s funny that you can Remove Soul a Soulless One but trying to pinpoint card effects to perfectly match flavor would a) be impossible to actually do and b) would lead to worse game play. Magic works best when as many cards can interact as possible.