Storage - once it's iced you don;t need to keep it airtight, the buttercream and icing 'seal' it and leaving it in the air will harden the icing. If you wrap it in foil you will get dents and creases in the icing.
A cardboard box (either a specific cake one or a normal one turned upside down and put over the top) is your best bet.
WRT buttercream and jam, you can use either to stick pieces together. Jam sticks better but buttercream is good because you can use it as a 'grout' to fill in small gaps. Same for sticking the icing on, you can get a more even surface with buttercream
A word of warning with buttercream - don;t make the coating over the outside of the cake too thick as you will end up with peculiar lumps and bumps when you put the icing on.
You don;t need to do a crumb coat if you're using fondant to cover it, crumb coats are really only for when you are covering with buttercream too, so you don;t get crumbs on the final layer.
Definitely slice the hump off the top of the cake then invert it so the bottom becomes the top, it gives a much flatter surface. You can fill in any gaps around the bottom with little sausages of sugarpaste.
Trim and fill both cakes at the same time so you can get them level.
Using fondant isn't tricky but does take practice. My top tip is not to roll it too thinly as it will tear and crack more easily as well as show every single lump and bump in the cake underneath.
You want to get it to about 5mm thick to cover well.
Knead it so it's warm and pliable it your hands, sprinkle icing sugar over your worktop and pat the icing into a evenish ball. Start to roll, making sure you go from right from one side to the other in one movement. Roll forwards and backwards then lift and turn the icing 90 degrees and go forwards and backwards again. Keep rolling, lifting and turning, sprinkling a little more icing sugar underneath every couple of turns. If it starts to go a peculiar shape, knock it back into a square with the heel/side of your hand.
The lifting and turning means 1) it won;t stick to the worktop and 2) you'll get an even thickness across your icing.
When it gets too big to lift and turn by picking it up, lay the rolling pin across the middle of the icing and flip one half over it then lift the rolling pin so the icing is dangling, then carefully put it back down and flip the top half back onto the worktop.
Use the same technique to lift the icing as you cover the cake, line up the bottom edge of the icing with the bottom edge of the cake and gently turn the rolling pin while laying the icing over the cake.
Use the palms and heels of your hands to smooth it into place, do the corners first, smoothing with the palms of a slightly cupped hand so they don't crack, then start to tuck the sides in. Try not to use your fingers as they leave dents (and take off rings too, I ruined several cakes with ring marks until I learned). Tease out any creases as you go round and try not to pull down as you do the sides as the top edge will crack and tear.
Be prepared to cover it more than once if you are a complete novice!
Your sugarcraft shop will have a board big enough - cover the board with fondant too, if you want a really professional finish.
Hope that makes sense, there are some good videos on You Tube showing how to cover cakes with fondant if you need to know.