First off, considering that rolling for stats is still the primary method of character generation according to the PHB, with stat arrays and point buy as options, the game is quite likely set up to take rolled stats into consideration.
Noticed that I used quotation marks around "rolled" ? This is because every single time someone shows up a character that he rolled the stats for, it has humongous stats (and it's usually with a "I was very lucky"). Once more, show me a character with rolled stats below average that has actually seen play. Because, you know, for every character that I've seen which was way above average, I should have seen one with below average stats, that's why it's called "average". But curiously, the ratio is more like 100 to 0.
Which shows that people who are interested in power will choose rolling, and will roll until they get something way above average. In which case, I agree that the floating ASIs become a minor point, although it's not the case, because with the even/odd bonuses, it might actually come in very handy to rack these few extra bonus points. So yes, very powergamy, all of it.
Secondly, as someone who has created or converted several hundred monsters into 5e at this point, the encounter difficult calculator is flawed because CR calculation itself is flawed. It only takes things that directly affect hp, AC, attack modifiers, and damage into account. Many conditions are outright ignored by the calculation. For instance, the only time anything related to being poisoned is taken into consideration is with a ghast's stench, even though poison can cause some serious side effects. Affects that reduce hp or stat maximums aren't calculated, which is why shadows and even the insta-kill intellect devourers are so horribly under-CR'd. The ability to charm, paralyze, stun, incapacitate, or cause unconsciousness in a target aren't calculated. Even petrifaction, which is basically insta-kill, doesn't affect outright affect CR according to the table on pages 280-281. The CR calculations also don't take monster weaknesses beyond damage vulnerability into account. There aren't many weaknesses in this edition, but they are out there (a yeti's fear of fire, a cyclop's poor depth perception). And, of course, there's that pesky action economy, which means that a party of 4-6 PCs will quickly destroy any enemy that has less than that number of actions.
The last point is sort of accounted for in the numbers of monsters in an encounter though. And about the above, the problem is not with the encounter calculator, my proposition is that you can never create one that takes into account all the circumstances of the fight, the synergies, and the luck. 4e was better because it was formal and restricted you to calibrated actions, but in 5e, it's silly to try. And that is a completely different subject.
This is especially true with the beasts, but it crops up with other creatures as well. I've gone through the calculation process a few times with a couple of monsters and came up with CRs that were a point or two off either way. It's possible that I've done my calculations incorrectly, but considering how many people say that the math is wrong, it's really premature of you to say it's because every one of their DMs allowed rolled stats (especially considering how many people don't use rolled stats these days).
WotC have their own CR calculator which is supposed to be more robust. After that, they tell you to exercise your judgement. Please do so.
And again, here's where you refuse to believe that anyone could ever possibly want to put that +2 in a stat other than their class's prime stat. I'm going to guess this is exactly what you would do, if you could, and therefore can't fathom that anyone else thinks differently.
Stat array is 15, 14, 13, 12, 10, and 8. We'll use that for simplicity.
1. Now, would you consider it "powergaming" if someone wanted to play a Strength-based fighter and put the 15 in Strength? If so, why? If not, why not?
No, that is perfectly natural.
(Mind you, the PH suggests the correct order is race, class, then stats, which means that it wants you to assign the stats based on what you want to play. The example character also indicates putting your highest stats in what your class needs.)
2. With that Strength 15, would you consider it "powergaming" if someone wanted to play an orc fighter with fixed racial ASI, and therefore started with Strength 17? If so, why? If not, why not?
Because he wants to play an orc, and actually an orc fighter. Orcs are usually stronger, it makes sense for them to be good fighters. And I will expect him to play an his character properly, in this case an orc.
3. With that Strength 15, would you consider if "powergaming" if someone wanted to play a smart orc figher, had Int 14, and put a floating +2 into Int for a total of 16? If so, why? If not, why not?
If he wants to play a smart orc, then he can put his 14 into Int, or even his 15, you know. And that is already a very smart orc, compared to the species Int of 7. Noone but the DM and him will ever see the stat. Why bother anymore, he can play a smart orc all he wants.
There is ZERO need for putting the ASI in there.
4. With that Strength 15, would you consider it "powergaming" if someone want to play a human fighter with floating ASI, anf therefore started with Strength 17? If so, why? If not, why not?
Humans are versatile, they are well known for it, so no problem. Because the choices are made for role and story reasons and not to get a measly +1 that will not matter.