MacOS – Mavericks – USB to HDMI Monitors

Mactracker lists a 2012 MBP as a 13 inch Retina model (MD212LL/A, MacBookPro10,2) with an Intel GPU and using up to 1 GB of shared memory.

Mactracker also says:

Supports an HDMI-compatible device while using one Thunderbolt display or support for two Thunderbolt displays

If the model you put down is indeed this one, then you'll need 2 Thunderbolt displays. Or you can run one monitor through the HDMI connection and another one through the Thunderbolt connection.

What this model can't do is run two HDMI monitors which it sounds like you're trying to do.

I highly recommend both everymac.com and Mactracker as sources of info on older Macs. Both have a wealth of information which would have given you the answer(s) you've been looking for.

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.