No, RNAi is against mRNA, which has already had its introns spliced out... Might be something else you are thinking of.
There is a good summary here:
http://fig.cox.miami.edu/~cmallery/150/gene/siRNA.htm
Yes, to get DNA into plant cells we can use viruses. One of them is Agrobacter tumefaciens, which I think forms tumors (galls) naturally on some plants. Or, a bunch of other ways... Since plant cells have tough cell walls, some of the techniques I normally use (I'm an animal person) don't work. But there are gene guns that fire DNA coated gold particles into cells, those will punch through a cell wall.
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2a) See rule 1
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