Dunno if you are aware of this (so ignore if you are) BT comes in 3 classes - Class 1 (supposedly) has 100m range, Class 2 has 10m range and Class 3 has only 1m range. I note that when I purchased a dongle recently I actually found it difficult to find Class 2 these days - this used to be the most common class, even the dirt-cheap Chinese no-brand models usually seem to be Class 1 now.
Either way, though, this probably explains some of the observations you are reporting. I think the first step is to ensure that you only use Class 2 devices. I, too, have considered the aluminium foil option! Haven't actually tried it yet, but I suspect that the carrier frequency is too high to have much impact. But I was considering moulding some kind of directional shield and earthing it to the PC chassis....