I just can't think the movie ever was conceived to be Decade's ending, in spite of Toei's statement about it.
There's no resolution at all to the "fusion of worlds" plot, with the solution proposed by Shadowmoon turning out to be a lie and then that plot thread just getting dropped.
There's also basically no following up to the whole Wataru thing from the first episode. Fitting it in the series is problematic, but I don't see how this movie could ever have worked as Decade's ending (Currently, I just go with the idea that they dropped ANOTHER wallpaper after the Rider War one initially appeared. Most things fit if you assume that...)
Anyway, answering the questions...
First of all, how does the movie link to the series??? I mean we saw di-end shoot tsukasa and then in the movie, it was like a totally different story!???
It happens some time before the ending of the series. After Amazon World and before Rider War World, somehow. Even though the series' footage gives no time for that.
2nd, Suddenly tsukasa become leader of dai shocker and wants to destroy em after he was kicked out? and wheree is wataru??? and also Blade guy????
Tsukasa WAS Dai-Shocker's leader before losing his memory, after recovering it he became the leader again, but clearly wasn't very fond of it. Look at his brief talk with Yuusuke during the tournament. After he had defeated the Riders in the tournament, he saw that he wasn't able to stop the fusion and destruction of worlds and Shadowmoon thought he wasn't necessary anymore, so he threw him out.
By that point, his whole reason for being in Dai-Shocker had been destroyed, so facing it was no problem.
And the riders that he fought, are they frm AU or the original universe????
Every Rider in this movie was made to closely resemble the original ones. The only ones that we saw without suits were portrayed by their original actors too (besides Kuuga, obviously). Take that as you wish.
Kuuga's ultimate form is kinda diff frm the series.........
In the series, he uses Ultimate form. In this movie, he uses Rising Ultimate form.
How come natsumi's grandpa is a dai shocker member??
Movie Wars 2010, the next Decade movie, explains a bit more about him, although it's still unsatisfying.
most importantly, how does any of this had to do with the series??
Dai-Shocker was the antagonist of the series in the last few episodes, but that's it.