I got given a 6 person dome tent which had no poles and tears in the tent/fly, so it was beyond salvageable as a tent.
I've cut all the material up from the fly, and have two colours, dark blue and a light beige. Pieces range from approx 2m square to small narrow strips of about 20cm wide. From the tent I have lots of black flyscreen and a bit of dark blue.
I'm using some of the flyscreen to make a flyscreen door for our bedroom door to keep the flies out but other than that I am stumped. I don't really want to chuck it out because it's in really good nick.
Never made bean bags before, but I would go the foam beans for the insides, it will give a nicer look overall and wont be lumpy [which can happen with stuffing]. You should be able to buy bags of the beans rather cheaply.
To go about making up the actual cover, I would design it out first-so you know what bits you need to cut and sew out. Then make up a mini version just to make sure it turns out the way it's supposed to, you don't want to waste metres of fabric making a full size version that doesn't work out!
How much of the pattern actually needs interfacing? I tend to find with most patterns you only need the interfacing for stiff areas [collars, cuffs, button hole/button spots]. Never really heard of a whole garment using interfacing throughout it.
I tend to trim the seams, depending on the fabric [experiment with a scrap of the material to see how close you can trim without ruining the integrity of the seam]. Corners I trim on the angle so they turn out nicely. And on curves I clip triangles out to help reduce bulkiness/form a smooth curved seam.
I'm often lazy and don't press, mainly because I have no ironing board!