Can I use a USB-C hub with a Type-A to Type-C adaptor to get HDMI from my Early 2015 MacBook Pro with no USB-C port?
No. An adapter with a male USB-A jack to female USB-C receptacle violates the USB specification. These adapters have no defined function in the spec and so run the risk of doing nothing, doing what you want and expect, or doing damage to your hardware.
The reason this kind of an adapter exists is because there are people that prey on the ignorant with cheap fixes to complicated problems. There's nothing wrong with ignorance except those that do not seek to end it.
There are USB-A to HDMI adapters but they use a GPU, not a protocol converter like most USB-C to HDMI adapters. You can know the difference in that the cables and docks with GPUs will require drivers to work, the cables and docks with DP-to-HDMI protocol converters do not.
USB-C is independent from the USB 3.0 spec. USB-C can be used for USB 2.0, USB 3.0, DisplayPort (DP), HDMI, Thunderbolt 3 (TB3), and more. USB-A only supports USB 2.0 and USB 3.0, to get video from it means using a USB GPU. Because these USB GPUs are using a protocol not intended for connecting a GPU they are known to be expensive, slow, and difficult to set up. Because many laptops with USB-C support DP natively on the port the cables are simple, low cost, fast, and require no drivers. There are GPUs built for computers that support TB3 on USB-C. Because TB3 was built for adding GPUs they are much easier to setup, fast, and while they are often not cheap they are worth every penny.
USB-C ports that support HDMI natively is possible, it's in the spec, but I have not seen it. There are micro-USB to HDMI cables that use HDMI natively (though more accurately it's the MHL protocol but it uses the HDMI connector) but these are also rare.
I have a USB 3 type A docking station at work that outputs DVI and Display Port that I use with my Mac, so I'm aware that my laptop will output video over USB, but I've tried the above configuration and the laptop sees the hub as USB 2.0 and no display is detected.
There are USB 2.0 docks with GPUs in them to get DP and DVI outputs. They work reasonably well for many people, they won't win any prizes for frame rates on gaming or video processing. USB 2.0 is considerably slower than USB 3.0 or TB3.
There's really only one way to get video from USB-A, a GPU. The most common I've seen is called DisplayLink. A confusing name as it is far to close to DisplayPort. DisplayLink is a brand name of USB-video GPUs. DisplayPort is a video protocol.
There are many ways to get HDMI from USB-C. There's the same DisplayLink GPUs as used in USB-A docks. There's native HDMI. There's DP-to-HDMI conversion, which is likely far more common than other options. There's TB3 GPUs with HDMI output, which is likely the most capable option. There's MHL on USB-C for video, including passive and active adapters to HDMI, but I have not seen actual products that do this, only spec sheets that indicate this is possible.
There is no easy solution here. I hope I've been clear enough on where the issue lies so you can decide on options.