Good advice. Maybe I will read up on these before trying again, if only to determine what kind of arena to build for each. I haven't found a summoning item yet for the twins, at least not that I know of (of course I wouldn't have known about the guide voodoo doll unless I had found it in the wiki).

You may have looked at a time where you had a penality, either from moon phase or time of day. The gear and bait modifiers seem to be basic +X, while the moon phase, time of day, rain and lake size modifers are percentage modifiers to the total.

I haven't looked all that closely, but that's how it seems to me.

They are much better, if only because they allow glide. But they also eliminate the need for the horseshoe. As you say, that requires Souls of Flight. Kind of a mixed bag. Learning to deal with wyverns gives you practice in how to deal with the Destroyer. But it is another step. Which is why I recommend buying them. And, yes, I follow my own advice there.

Yes, they function similar to the boots, but you get to use both. You get a second and a half or so with wings, plus the boots. And if you get good with the Ice Rod (from the Wizard NPC) you can place a block in mid-air, hit it, and restart the counters for both, which gives unlimited flight time.

If you don't want to mess with all that, the Gravitation/FeatherFall Potion combo also works well, so long as you can deal with "down" changing all the time.

Cutlass is awesome. Pretty much what you need until you get Terrablade. Reforge it if you didn't get a good one. (Full disclosure: I rarely reforge it. I just summon pirate invasions until I snag at least a Godly. Sometimes that means enough to craft a Greedy Ring.) Hang around the surface whenever it's raining until you get a nimbus rod. They make things SO much easier. (Hint: If you place the cloud on top of a single block, it will rain THROUGH that block...)
Could well be. I only checked a time or two, and decided that even if for +15, it wasn't worth my while to carry them around and risk being almost defenseless when I inevitably forget to change back to Meteorite/Titanium/Beetle.

In fact you can copy the world file before destroying each of the first three altars. The choice between cobalt and palladium is randomly made when destroying the first altar, mythril or orichalcum is only decided when breaking the second altar, and it's not before you destroy the third altar that the world will set for either adamantite or titanium.

Dragging this up from the depths to re-mention fishing to get crates. HUGE help to have a big stockpile available when you get into Hardmode. You might be fishing a lot anyway to get the components for the PDA (I certainly did), so you may as well use the Crate potions while you're at it. I don't bother with the Sonar potions, though - bait is so easy to come by that I simply take 50+ pieces of bait with me and sell the regular fish for a tidy profit.

I was hoping to get the same results from dropping Silt, etc., into the Extractinator but you only get pre-Hard stuff there. If you stockpile Silt / Slush then either burn it up before going Hard (it still works but I find the drops are less useful at that point, except Amber and maybe a couple others) or save them to create buffers around Corruption / Hallow / Crimson.

The Nimbus Rod has been mentioned a few times. Great weapon, especially for segmented enemies like worms and Wyverns. The rod has also been handy when dealing with a Blood Moon: the clouds will kill a lot of the weaker stuff outright, and they'll soften up the rest for you. Handy when the numbers start to get a bit overwhelming, which can happen easily enough if you've only recently killed the WoF and haven't yet found much Hardmode hardware (which is where all of those crates will come in handy, giving you instant access to a bunch of the Hard ores).

I previously thought the Medusa Head was also a bit useless but I've since played around with it a bit and find it to work well in concert with the Nimbus Rod when dealing with Wyverns. Set up a couple clouds, and then attack with the head. Both will damage most of the segments during a Wyvern attack, making it easy to take it out within a few passes.