It's hard for me to visualize what you're saying about the TIU connections. Perhaps a photo would be illustrative.
It's possible to control conventional locos with the DCS remote and the TIU's variable channels (if your TIU is fully functional!) I did it myself a long time ago but from experience I've concluded that it's best to run each type of loco with its native control system: PS2 / PS3 using DCS; TMCC / Legacy using a CAB-2 and Legacy base; and purely conventional locos using a Z4000 or better still, a venerable postwar ZW with appropriate circuit breakers and transient voltage suppression!
It sounds like you already have several hundred dollars invested in the hobby. No matter how many locos you ultimately collect, you only need ONE of each brand's control system. Thankfully they're interoperable, if not fully compatible. I.e., some features will not be available with the competing brand's hardware. Good postwar transformers have become quite affordable, so my advice is to go that route and avoid using the TIU in variable mode. Just my $.02, and sorry that I wasn't able to answer your question more specifically.
Please see also this thread: https://ogrforum.ogaugerr.com/topic/power-for-post-war-engines
