Agreed on the inputs, as noted earlier, portables tend to be high impedance. It is cheaper to just dump the attached whip and the external antenna jack into same frontend. Kind of like lots of portables skip on even basic $0.25 ESD protection diodes.
I have used symmetrical feedlines for various antenna projects, though just from a convenience standpoint alone, I suspect there are far more SWL enthusiasts here with dedicated MW/HF antennas running coax instead of twin-lead, ladder-line, or similar. ;)
Either way usually even a basic outdoor antenna has more than enough gain for a portable regardless of whatever feedline impedance characteristics barring a short, and even then that might work depending upon the frequency.
We know he has a seemingly working active antenna available, so that is a potential place to start before delving deeper into antenna design. Assuming it is not the active antenna preamp itself overloading, adding attenuation is the usual next basic step to deal with frontend overload.
Coax is present so I lean towards us starting with readily available and affordable attenuation options; be it a variable attenuator or stacking fixed attenuators if need be. Various F- and SMA-connector models are available for a few dollars at a variety of online vendors and electronics shops. Add a couple of cheap adapters if needed to insert the attenuator(s) between the bias tee and radio. Alternatively, three resistors can do the same if one does not mind a little soldering DIY job.
https://www.everythingrf.com/rf-calculators/attenuator-calculatorThat said, this is not exactly ideal....
miniwhip was working without amplification - as a passive antenna
....as Kirchhoff's current law applies. ;) So just like any other dipole antenna, even if your setup does not look like a traditional one, you could consider the radio circuitry (plus any electrical grounding, plus possibly the whip antenna, etc.) as one side of the antenna, and the coax running to your un-powered active whip as the other side of the antenna. Lots of YMMV there. It very well can and should receive signals, but I can imagine the resulting SNR is not great.