Can a voltage divider be used for (input) jack detection?
TRRS jacks with a detect switch are not at all uncommon if you go to an actual electronics supplier, they may only be seeming hard to find if you are shopping hobbyist offerings on general ecommerce sites.
Doing electronic detection as the receiving end of an audio signal will not be easy, as you know extremely little about how the source works.  For example, you cannot really use a DC test current from a pulling resistor, as don't know if the source presents a DC path at all, or if it relies on a DC blocking capacitor.
In theory, if you wanted to badly enough, you could implement some sort of threshold detection and pulse stretching with a comparator or MCU ADC input which would see the presence of an AC signal.  But the problem with this is that it would treat silence as unplugged - which is probably not what you want.
If you absolutely had to do this electronically, you could do something really fancy, such as periodically injecting a weak ultrasonic AC signal and seeing if there was a source impedance loading it, but that's getting to the point of absurdity when there's a simpler way.
Realistically, just buy the readily available correct part, with a switch, and move on to the interesting parts of your project.
That's almost certainly what existing wired/BT dual mode speakers on the market do.