Things about CRs:
1) I will be using mostly a very low variation for CR. For example, I told one person (you?) that they are facing an Aboleth as a threat. That's a CR 7, but against a party of level 6 characters. So to compensate, at least one encounter will be EL 5 or lower. Though I could argue telling him "Aboleth" beforehand justifies the +1 CR by itself, I'm not going to.
2) For solo members, it may be a single encounter of EL equal to their level, or two encounters of EL equal to level -2, or four of EL equal to their level -4. With the normal variation. You will find this is all according to the Encounter guidelines for reduced party size. Additionally, for parties of four, it is technically within my rights to pit them against a single encounter of EL equal to their party level +4, though if I do so, it will be rare.
3) I am going to be using, for the vast majority of fights, monsters, not NPCs. Some monsters do cast spells, or use SLAs, and they are usually some of the tougher monsters. But I am not trying to abuse the CR system, or use four level 12 wizards going nova from ambushes as a demonstration of the fact that fighters can't face equal CR encounters.
4) All the encounters are made ahead of time, because I have a long list of "generic EL X encounters" that I have made for most levels. If you pick a level I don't actually have prepared (hint anything above 13 falls into this category) then I will be forced to make them then. However, I will definitely have them all finalized before I ask anyone to prepare spells, and hopefully before I look at in depth sheets. That said, it really is within the DMs normal duties and actions to dictate who fights the party based also on their characters, such as not sending Shadow Hit Squads against people with no access to Magic weapons or Ghost Touch effects, and to have plot involved hit squads who have some knowledge of the PCs past tactics, so when I pick challenges that I think are good challenges for the party from my list, that's not good complaining.
5) My goal is not to defeat the party by any means. My goal is to show that high powered characters are actually pretty well balanced against appropriate CR, and that smuck classes are actually weaker than is needed to face them.
So for examples:
a) A party of Fighter/Dragon Shaman/Factotum/Favored Soul, my "goal" would be to defeat them, and make them all look bad. Because those are smuck classes.
b) A party of Wizard/Druid/Wizard/Cleric, my goal would be for them to survive all four encounters, but have had some genuine serious tension in most fights, and generally feel like "Man I don't have another one of those in me" after the fourth one.
c) A party of Wizard/Druid/Cleric/Fighter, my goal would be something like b, but with a strong does of "Jim was not particularly useful, and I kinda wish we had another Bob instead of him."
6) "Situation modifiers" No, an "ambushing" Gelugon is not of higher EL. A Gelugon has at will Persistent Image, amongst other things, and the fact that you might run into a Persistent Image den, and the Gelugon detects you before you detect it does not make it a higher EL. If you can't deal with the abilities of an appropriate CRed monster actually means you can't deal with the abilities of an appropriately CRed monster, not that it's unfair for a Gelugon to use Persistent Image. It's also not unfair for it to fly, because it has fly as an at will SLA.
Likewise, to quote from the blue Dragon combat description: "Typically, blue dragons attack from above or burrow beneath the sands until opponents come within 100 feet. Older dragons use their special abilities, such as hallucinatory terrain, in concert with these tactics to mask the land and improve their chances to surprise the target."
If a Blue Dragon surprises you, it's not a higher EL fight. Because that's what they are expected to try to do.
Also, fighting a monster in it's natural habitat does not increase it's EL either. Fighting an Aboleth or Water Elemental in water is not higher EL than CR X, it's CR X assumes it is in it's natural habitat.
And for double sure choosing to be a party of commoners is not a situational modifier that gives you a pass on fighting level appropriate challenges.