Running full time four wheel drive will make the car more stable under braking. On the 4141mII you will need to buy a front diff and a locked centre pulley, the TA04 ones are fine, a lot cheaper and you can buy the delrin outdrives like the ones in the rear of your 414 later if you want too.
I would, however persevere with the one-way, unless you are racing on a small and low grip track, it's not bad once you get the hang of it.
As for the rear spinning out when you floor it, that's nothing to do with the one-way. Under power the car is four wheel drive. Make sure your rear diff is set correctly, not too slack or tight, get an experienced racer to have a look. Also try to be a bit smoother on the throttle, you wouldn't expect any car to stay straight if you floor it on the apex of a corner.
Your tyres & inserts should be fine, ask around and see what others are using at the track and go with that. I find firmer inserts better.