The effects of time dilation are sort of explained in the rule books, and yes a PC or group of PCs can go to a fast time shadow spend 10 to 20 years training and then come back with enough skill to have an advantage in a given situation, with only a month or two having elapsed in that shadow (especially if that shadow is Amber, remembering that by cannon time flows 10 times faster at the Courts than it does in Amber). That is not to say they get a free ride. Whilst I manage what rates of time streams are available, characters can only gain and/or develop skills, powers, abilities if they have the CP available to spend, and they only get CP from progressing the story line in some manner. Now this progression may specifically relate to some goal of the character, or another character, or it may actually relate to some element I'd set up in terms of plot.
In relation to the "Elder Amberites" or at least those still around in my campaign they as a base have about 50 times the CP of the PCs, so a PC taking on one of the Elders has never arisen, even my NPC intermediary(who has about 5 times the CP of the PCs) generally won't mess with the "Elders", but that doesn't mean they're not prepared to stick 2 fingers up to them, ignore them, or be generally uncooperative.