First, make sure the head of your "new" 424 is in good shape. Tape heads can go out of alignment, which will significantly reduce the quality of the signal you are able to get our of your tape.
Next, even if you can get your system to recognize a pair of 2-channel recorders (easier on OS X with its Aggregate Devices than on Windows), they still won't be ...
Mic positioning is part skill, part art.. & also part guesswork.
I could tell you that you need the mic approximately 6" above the bow, 2ft in front of the strings & pointing at an ƒ-hole... but that's just because that's where it gave the sound I wanted last time I did it - in a different room with a different mic & a different player on a ...
The speed may vary (and actually you may even see differences between units of the same model). If you know the tuning, chords etc you can always go through a computer and resample if you want pitch to be perfect.
You can use Sony - Sound Forge for precise pitch-shifting with curves if your problem is a problem with the tape recording speed.
To note that pitch-shifting if no time-stretch is added to it doesn't significantly alter your original sound quality it will only change the tone.
Steinberg - Cubase's tool seems pretty accurate too.
Also, Ableton - Live and ...
The way mp3's are encoded and compressed doesn't lend itself well to this at all, so you need to convert to wav anyway. The way I would do it is to script up the following:
convert mp3 to wav
pipe wav into FL studio
run the plugin
export as mp3
(if the FL studio steps can be scripted too, all the better, but I don't know the app)