Lower stack stem and headset are the only save solution. But i don`t think that will be enough. If your a little bit brave i would suggest getting a propper shim and only using the lower of the two clamping screws on the stem. This will hopefully have the same effect as the usual spacer on top of the stem. Regarding the top cap you can do what you want as it is only used for adjustment and not supposed to carry any load while riding.
Lower stack stem and headset are the only save solution. But i don`t think that will be enough. If your a little bit brave i would suggest getting a propper shim and only using the lower of the two clamping screws on the stem. This will hopefully have the same effect as the usual spacer on top of the stem. Regarding the top cap you can do what you want as it is only used for adjustment and not supposed to carry any load while riding.