Hi Fluff,
I used to work as a ski boot fitter for about 3 years whilst at University - hopefully some of this will help:
Firstly your above post appears to suggest that the underside of the heel of your footbed is curved - this is wrong. The footbed should be fitted with a heel stabiliser which is then ground (using a sort of belt sander) so that it is flat.
I.e. the finished article should NOT look like this:
[
www.tallingtonlakesproshop.com]
But should look something like this (note the grey foam which sits under the arch and around the heel):
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stk.tetongravity.com]
If the footbed is not stabilized then it will be doing absolutely nothing to help you and may actually be encouraging more movement in your foot.
Secondly if they are in fact stabilized then make sure they are trimmed to the correct shape and size - you can do this by comparing them to the standard footbeds that came with your boot - if they are longer / wider than the standard footbed then they will not fit properly and will reduce the boot volume.
Thirdly, and I do not mean to be patronising, but make sure you are putting your boots on properly. When you put your boot on you ideally should be sitting down, open up the shell by pulling the tongue to one side (not directly forward), put your foot in and tap the boot several times on the ground on the back of your heel. This makes sure your heel of your foot is sat properly within the heel pocket of the boot. Do up the boot starting with the lowest buckle on the cuff (the vertical part of the boot). Tighten up the cuff as tight as you are comfortable with and tighten the powerstrap. The buckles on the forefoot are pretty much purely there to make the boot watertight, they do not need to be tight - overtightening these can alter the shape of the entire boot. It should take around the pressure applicable by your little finger to close them. Much more and you are likely overtightening these.
Fourthly - heat moulding the liner is not really anything special or as technical as it sounds. The foam in liners simply becomes slightly more suggestive when warm and thus the bedding in process can be accelerated. It will not radically change the fit of your boots. If you do get your liners heat moulded MAKE SURE that the footbeds are taken out of the liners whilst they are warmed up (as prolonged exposure to heat will cause them to soften and change shape). You can put them back in as soon as the liners are taken off the heater. Put your boots on as normal and then spend around 10-15 minutes gently flexing the boot. This generally will achieve the same results with respect to bedding in as 2-3 hard days skiing.
Fifthly if you have to wear footbeds in your other footwear (trainers, golf shoes, walking boots) and have to see a podiatrist for them it would be worth getting an opinion from them. Very few boot fitters are medically qualified (although there are a number of notable exceptions who are industry leaders), so if your problems are serious it's worth getting a second medically qualified opinion.
Finally some people do experience pain even with a really well moulded footbed whilst their foot gets used to it. Personally my left ankle is really unstable, and even though I have an absolutely textbook footbeds from one of the best fitters in the country I do occasionally still get some grief from the arch in my left foot. For recreational skiers discomfort should be minimal, but for someone who pushes their skiing hard and has a tighter fit and aggressive footbed then a little bit of pain can often be invariable - sometimes there is a sacrifice to be made in terms of comfort in order to get the best performance out of your ski boots. All that said it sounds to me like there could be another cause for this, so take them back and get them re-done or take them to another shop / podiatrist for a second opinion.
One more thing - once all fitted up and sorted out if you find they are still giving you a little bit of grief ask your bootfitter about the possibility of using heel raises / heel lifts. I've always found them to be a bit of a cure all in terms of getting the foot-footbed love affair off the rocks and back onto the road.
Anyway hope all this helps.
G