Glad to see that you got the line color figured out, Sarah. I'll help here with the curving lines and arrow sizes. Here's my workflow:
I created this sample csv file, routes.csv, which consists of three routes (a, b, c):
Note that each route contains four fields that define its starting and ending locations, each in longitude, latitude order. I've also added a arrow_size field (more on that later).
I added the csv file to QGIS. Then I loaded the Shape Tools plugin (Plugins > Manage and Install Plugins > Shape Tools). After installation Shape Tools appears under Vector > Shape Tools. (I'm not sure which QGIS release Shape Tools will work under. I use 3.16.1, so if you don't find it at 3.10 you'll need to upgrade your QGIS version)
You'll notice that Shape Tools contains a variety of tools. I used XY to Line, shown here.
The Input layer is my routes.csv. Line type Geodesic creates curving lines. The starting and ending X/Y fields are from routes.csv. FWIW, my output lines and points were stored as Geopackage layers, but you could use shapefiles or temporary layers as output.
Here's the result, with the curved (Geodesic) lines (For the background, I used the Natural Earth 1:110m coastline layer):
Now, about the arrow sizes... I used the QGIS concept of Data-defined overrides, where the input data defines the symbology by overriding the default settings. So in my example, the arrow_size field contains values which will be input as data-defined overrides.
In Layer Styling, I added a new line (via the green plus sign), which I changed to Marker Line (drop-down), then checked on last vertex only. I also moved this new line type to the bottom of the symbol list, using the blue down triangle to the right.
Finally, I set the marker size by: a) clicking on Simple Marker, b) clicking on the box-like symbol circled in red (that's the data-defined override symbol), c) checked on Field type, and d) checked on my field arrow_size. I also rotated the symbol by 90 degrees (not highlighted). Voila! The markers are now sized according my my input data values.
Sarah, in closing here's a Stack Exchange Pro Tip: SE prefers that submitted posts contain just one question, so that future users can concentrate on searches specific to their area of interest. Your post contained three separate questions, which probably should have been submitted individually. I know that might seem cumbersome, but that's the SE model! All the best.