1) Which ev spread is the best one for Blaziken's both normal and his Mega forms? Well it be 252 for Attack,and Speed with 4Evs for Health?
Definitely. Blaziken is meant to hit fast and hard, not stay around to take hits so that spread is ideal.
2) Do you think that Pokemon of the Week/Day can be very helpful for the competitive battles?
To a certain extent, yes, most of the sets and analyses are geared towards competitive battling. However, there are more in depth sites out there as well.
I was thinking of having this set for my perfect Blaziken and his Mega form. Flare Blitz and High Jump Kick are his main STAB. Sure Sword Dance can boost his attack sharply. But Hone Claws can boost his attack and Accuracy at the same time. That way I wouldn't have to risk his High Jump Kick,and it can't be missed. With Speed Boost, it can outspeed about everyone one. With 160 HP, it will become not only strong,and fast,but it also become bulky as well.
EV: 160 HP/ 252 Atk/ 96 Speed
Moveset:
Sword Dance/Hone Claws
Flare Blitz
High Jump Kick
Protect
What do you think?
Hone Claws is actually not a bad idea. The only problem I foresee is if you meet a Jellicent or some other wall, where Swords Dance may be of more help. As for EVs, it doesn't matter that much (IMO) since Blaziken won't be taking much hits anyway.