As we have mentioned before, you can flatten the arch in almost any shoe or boot by adding tape or adhesive foam (e.g., moleskin) under the insole (or over it, but you want to be sure the result is smooth, so it doesn't injure your foot), in the places where you want to flatten the foot bed, or custom cutting your own insole - provided there is still sufficient room for your foot inside the boot after you have done so. It is an easy thing to try, and it costs very little. Likewise, to some extent you can widen the boot at any point, including the toe box, with something like
this - e.g., if a side pinch is forcing your arch up too high. (I'm not sure that would work for synthetic boots, like some Edea.)
I suppose that if you get a very low heel for your anatomy, that too might cause problems. E.g., it may be too hard to point the toe pick into the ice sufficiently for some figure skating moves.
Some boot making companies probably ignore the detailed shape or tilt of the underside of the foot - e.g., I don't know whether the common laser scanners pick that up. Fit is further complicated by the fact that putting your foot into the boot, and putting your weight on it, changes the shape of your foot, and some people need the boot to change that shape in particular ways, in order to be comfortable. Also, for some reason, boot makers are not completely consistent when they manufacture boots. Finally, at least for ladies, fit can vary with the stage of their feminine cycles. Thus, even if you get custom boots, it is not uncommon for people to feel a need to make additional adjustments, similar to those discussed. (I'm not sure, but maybe some people even use different insoles at different stages of their cycles. Incidentally, if you are female, and will become pregnant soon, now may not be the best time to get custom boots. For that matter, any major weight gains or losses, can cause foot size and shape changes too, in both men and women, sometimes permanent. )
Regardless, it is worth contacting any custom boot maker to see if they provide a guaranteed fit that makes you happy, how that guarantee is activated, and what you would need to do to get them to make good on it (e.g., travel to their factory again, within a few weeks). I personally wouldn't get a high priced custom boot from boot maker who wasn't willing to guarantee satisfaction.
If you are indeed reasonably close to the Avanta factory (note that they also sell through dealers, who might have different policies), last I knew (I may be out of date), I think they had the advantage of including a free consult with their own licensed podiatrist who looks at and measures your feet, when you buy custom boots, if you come to them. Since you are having trouble figuring out what you need, a medical professional like that might be helpful. It would be really awful to go through all this trouble and expense, and still end up with boots that didn't work for you. Being close to Avanta might also mean you could afford to travel to them for a second meeting to make the final adjustments, and check the fit. Were I you, I would also look into whether there are cheaper ways to travel - e.g., drive, bus, train, carpool with someone else at your rink who wants custom skates. But it's pretty scary to buy anything from someone who doesn't even bother to list prices on their website! "If you have to ask, you can't afford it."