either people can buy it, or it's open source. These possibilities are exclusive, you can not do both for the same thing.
I have never even seen the TRS midi version. All i see is either 5-pin Din or USB-midi, which isn't midi-really, but that exists and can be done with an Arduino, though preferably with a 32u4 based board.
Merging i have done, but the timing can be an issue. Most midi-devices do not send more commands than can be transmitted. At 312500 Kbps, 3 bytes take a little less than 1ms to transmit. Midi is a command stream, so any merging will mean you will cue those 3 (or 1 byte) commands for several inputs. Now if there is a constant incoming stream on both ports you may have some issues. Also for reliability, a UART (hwSerial) is definitely preferred, so you may want boards with more than 1 UART.
I recon it's 3.5mm TRS we are talking about here, you know if T was 5v+ and S was data, and R would just simply not be connected, i wouldn't even mind so much.
Well the issue may not be all that problematic. 5-pin Din used to be the most common Audio connection plug when Midi was originally specified. the switch to RCA only started a few years after that. So the current limiting resistors on a midi output probably suffice, and probably on the input as well. (this is for 5v specs of course .. 3.3v may be different.)