You can upload sketches via the RS232/TTL level adapter (vs USB), but will need to press reset as part of the upload process when the IDE shows "compiled xxx of 32xxx bytes". Don't try and send to the card via USB while sending to the card with RS232, they will corrupt each other. Both can receive from the Nano at the same time.
The 1K resistor provides a little bit of separation - when USB is not active, it outputs a high level so it acts like a 1K pullup resistor on the line. If you connect the RS232 output with 1K also, when it is not active it will look like a 1K pullup to the USB.
5V with 1K pullup means the whatever is driving the line needs to sink 5mA to pull the line line, and the next to nothing to bring the line high. The 16U2 can sink 5mA no problem. The RS232 driver then needs to sink 5mA also.
Check the datasheet for the part you want to use.
A lowcost MAX232 such as this one
see page 5, 7.7 VOL, will pull the line to 0.4V with 3.2mA.
Other devices will have different characteristics - mostly worse from what I can find!
At 5mA, the 0.4V may creep up some - as long as it stays under 0.3 x Vcc, 1.5V, it will be seen as a valid low.