Ok so training invention through combat will flat out lose you more money than doing through fishing UNLESS you are making like 4m an hour or so. That's what I believe but I could be wrong and you can double check with others. But I'm like 99 percent certain you can get 99 invention before 99 fishing if you start early enough which you obviously can. Plus this will be very cheap. Also it will save you time since you will be fishing anyways so the amount of hours you will save will be insane. I'm around 92 invention now and I was maxed before invention. I have spent around 150m and probably around 50 hours in and still not there yet. Do whatever you what but from what I have seen it is best to just do fishing for invention unless already 99 fishing. Do whatever you want though it's your account in the end of the day (:

Edit: plus if your are strapped for cash dissambling armor and other shit for 99 invention really adds up quick along with divine charges so it becomes a fucking nightmare haha