15-80mm/s, depending on the model and material.
Absolutely. I can print PLA at 3x the speed of PETG for an equivalent level of detail. Print PETG too fast and all hell breaks loose...
Color... yes as well. I have some clear and opaque colored PETG from the same manufacturer that have distinctly different printing characteristics. Remember that each material has a "natural" color, and anything else is the result of additives. Anything added to a filament will change its characteristics. This is particularly true when going between transparent and opaque colors.
//edit: Also, don't assume that each spool of the same material from the same manufacturer has the same filament diameter. Everything is done in batches, and on different machines, so there can be variances from batch to batch. I have two spools of PLA from the same manufacturer, one of which is just a hair under 1.75mm, the other is a consistent 1.78mm. If I use the same settings when printing them, one will be noticeably over-extruded - rough first and last layers being the most noticeable impacts.
In some cases, bed heating (or rather the temperature change when cooling) can be used to ease part removal. So you can print on a hot bed and have things stick well, then have those parts pop off nicely with very low effort when the bed cools.
Having a heated bed is also handy to keep your room warm in the winter.
Yes, material and part depending.
High-detail prints on PETG will use the fan, as will PETG bridging. But layer bonding strength suffers. Lower detail parts, or parts which require more mechanical strength, will use little to no fan.
"All of the above".
No need to print cubes at the like to re-calibrate X/Y/Z scaling, but you definitely need to double-check the extrusion width, temperatures, and perform the test prints to re-calibrate your linear advance K-factor. K-factor is material-specific - that's why it's best to set it via G-code, not hardcode it into the printer firmware.
Advice for anyone new to 3D printing- LEARN 3D MODELING.
The joy of printing trinkets and other designs from Thingiverse only lasts for so long. The real fun from 3D printing comes when you imagine something that doesn't exist, design it in CAD, and then watch your creations materialize on the printer.