Not weird? Not really sure what the question is there. A fairly economical computer, one fan is enough for most use cases, not even really necessary for most of what I see people use that class of computer for.
You can mount a fan anywhere you please... double sided tape, drilling your own holes, fishing line, jamming a sheet metal screw through one of the grill holes, lots of options.
Not sure why you would want a front and rear exhaust. Would lead to a negative pressure arrangement and the incoming air would take the path of least resistance, probably out the back and the front wouldn't be doing much at all. Typically front is intake and rear exhaust. Side panel intake isn't strictly necessary, but you could do a positive pressure airflow setup with that.
I've owned a few Optiplex over the years, currently have a 9020 chassis and power supply sitting under my desk. A nice i7-4790 I picked up cheap. At work we've had them since they came out, swapped in SSDs and they run Windows 10 now. Mostly SFF models. Couple we added ram to for local VMs. I suspect they'll start dropping like flies soon (power supply is usually what goes). But I think we have enough on the shelves to keep them around for a few years yet.