Not all enlarging lenses have flat fields. Leicas are the biggest culprits. It is a trade off to keep astimatism under control and they do it with all their lenses.
The other probably bigger culprit is not having a dead flat negative. Even with a perfect lens, only the center or the corners will be in focus. I have had some sucess matching curved field lenses with non flat negatives. The defects seem to cancel.
Best results are with a true flat field lens and double glass neg carrier.
There is no way a 1/8" bow in the center of an easel can put the image out of focus and there is no way you can have a bow that big.
The double bubble level is ok to get the lens stage to match the base. It is not accurate enough for the lens. You need a lazer device or one of the other products for this. I just use my Peak 10x grain mag at 16x20 and match left/right sides, then forward/backward. After you do this, put it in the corners and all four will be sharp at one time. Then work on the center with a proper lens and glass carrier.
BTW, every lens is different. So you can`t unscrew one and put another one in it`s place. I know it`s more than you wanted to hear.