MacOS – Will one USB-C port be able to deliver power, data transfer and power two monitors on a Macbook Pro

The MBA GPU may be limited to a single external monitor, but the USB to HDMI requires a driver installed on the MBA and that video/image is not using the GPU, it's using the CPU. So the limit is bypassed. Now it might be more complicated than this, but I think is this the basic premise. Be aware that for fast moving videos or games the USB connected display may disappoint. Also if the MBA is running near 100% CPU usage and then you add this USB display it may also disappoint. If you looked to add more screen for email, YouTube it's likely to be fine. The hub might be more of an issue, if the cable is USB 3 make sure the hubs too

I would go for USB-C to DisplayPort directly. That would be one relatively inexpensive ($20 to $40 US prices) cable to do the job. Since your display won't charge the MacBook and do video over a single USB-C cable and you have several ports (display side and Mac side) - a dedicated cable would be my choice.

I like the USB-C to HDMI adapter you mentioned for my MacBook since there is only one port and charging plus USB is needed, but on the MacBook Pro you have several ports and can use one per display without too much hassle. It will work for you, though - just not my first choice.

Apple even recommends against using the adapter you mention for bridging between a Mini DisplayPort device and thunderbolt 3. - https://support.apple.com/en-us/HT204154

Note that although it uses a Mini DisplayPort connector, the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter doesn't support connections to Mini DisplayPort displays.

If you only want two external displays, both 13 inch models of MacBook Pro are suitable. You'll need the 15 inch models to support 4 displays. That might be a non-issue if you only have one Dell display to connect.