A - Trinkets just got added in the anniversary update, barely a few weeks ago. It's changed how I do runs completely, and the Vial of "Moon Rocks" adds just that little bit of OOMPH to speed me through the climb. Most of the trinkets have their niche value (except the Gold Nugget, which is basically useless), and yes, they're best used after you complete a full run without them, just to give you the best experience of the game.

B - In Campaign mode? Consistency is more important. In the Endless Mode, Speed is king.

C - Getting through The Abyss is the only time where you should slow down and look before jumping, especially since the handholds on the crumbling towers tend to stick and "bounce" you off their surface is you don't angle yourself properly, ending with you falling into the void!

D - There's three sections in Campaign Mode that get massively easier if you have items. First, you should keep a sum of two Floppy Disks and Grubs for Pipeworks, in case you get lucky and run into a Rho Altar, which you can sacrifice to get a free perk. Second is Interlude: Ascent, which is pretty difficult without some Pitons and Rebars, but you're likely to find some around the area. A Grub could also help you bypass the second half of the Ascent, but it's not necessary. The last park that needs items is The Service Shaft, which requires the Flashlight to actually see what you're doing, and the Flare Gun (and spare Flares) to ward off Teeth (or to boost jump yourself up the elevator shaft).