If I accidently ev trained my pokemon in hp and it has 255 in hp, then I gave it a pomeg berry(Reduces base hp), will the 255 go down 100?
Yes.
1 Berry = 255 -> 100
2 Berry = 100 -> 90
3 Berry = 90 -> 80
4 Berry = 80 -> 70
And so on, 10 by 10 until 0. If you're Pokemon has 101 HP EVs, the first Pomeg berry will still reduce it to 100, and then you'll need another 10 to lower it to 0.
The first berry lowers your Pokemon's EV to 100 if it has EVs OVER 100. If it has something like 95 HP EVs, it will be lowered to 85.
Technician works on each attack of fury swipes?
Yes, however, it's not a very good attack to use Technician with.
Fury Swipes is 18 -> 27 per hit.
27 x 5 = 135 Base damage. Multiply that by .8 because of accuracy and that equals 108 average damage.
Return can max out at 104 Base IIRC and never misses.
Double Hit is 35 -> 52.5 per hit.
52.5 x 2 = 105 Base damage. Multiply that by .9 because of accuracy and that equals 94.5 average damage.
Despite this, it's still popular to use Double Hit instead of Fury Swipes and even Return on Technician Pokemon because Double Hit has greater accuracy and Fury Swipes stops at the first miss. Fury Swipes would be better if it hit all 5 times even after the first miss. Example: Miss, Hit, Hit, Miss, Hit.
Double Hit also is much more reliable at taking out Pokemon who use Focus Sash and those who use Substitute because of the accuracy.