wayoverthere

No, they're not natively set up to be paired into a repeater like the kg1000g.
Could it be accomplished? Sure, with some custom work on cables. The hard part would be getting the audio from the receiver to the input on the transmitter, and and handling the ptt. I don't Remember  if there's a VOX setting or not. I don't know if there's any off the shelf cables that incorporate some or all of the adaptations needed.

Diamond rates this one at .1 db insertion loss, and 60db of isolation between ports.
https://www.dxengineering.com/parts/dmn-cx210a
I have one on my desk currently; when my faltering btech gets retired, I'm planning to drop it in between the diplexer and my uhf radio, so I can switch between the feed to the dual band ham antenna (comet gp1), and the gmrs antenna (Midland 3db whip)

Sort of a blind guess, since I don't have any experience with the CDMs or the rim-maxtrac/RPI setup. With what works and what doesn't , it sounds like the issue is in the audio path from your rx radio. It's clearly hearing and keying up the tx, and that you can hear on the Rx radio itself says tones probably aren't the issue. 
Is it feasible to shuffle the radios, use the tx radio as the rx, to rule out an issue with the Rx radio? I'm think if there still no audio, the issue is between the radios, cable or a setting in the rpi.
One other thought is if there's something in the programming that needs to be done to enable audio out on the CDMs.

i would imagine a simple headphone splitter would work, if you have a recorder with a 3.5mm input; feed the other side of a splitter to an external speaker, and good to go.  alternately, some fiddling in the settings could probably work to feed the audio into a pc and have it both record and play through the speakers.
i currently have my two base radios fed into a y connector (meant as a splitter, but i'm using it to combine), which then feeds either a pair of computer speakers through their harness, or into a headphone amp and headphones (cuts the volume level for late night listening).

You don't need to mess with simplex channel 18 to use the repeater, just RP18. You CAN do it in the software if you want to, but if all the channels are as shipped, you can do it in the menu.  you just need to tune to RP18, go in the menu and look for "t-ctcs" or "t-dcs", menu to select, set it to the correct tone (provided by the repeater owner), and hit menu again to confirm.

For receive tones, think of it as a filter on what you hear. No receive tone set is the equivalent of no filter.

Your 275 (like other repeater capable midlands) comes preprogrammed with channels with the correct offset for gmrs repeaters (+5). You may need to enable the repeater channels in the menu; once enabled they will show up as 15RP-22RP.