I expect you'll get a raft of peeps saying that Scat shifters are shit and you should buy a Vintage Speed (which break) or a Gene Berg (which are very expensive). I like a T-handle so bought a Scat, and had exactly the same issue as you, went into reverse without the trigger pulled up, crunching into second, infuriating. I spent a bit of time stirring the cogs while looking at the actual workings of it and found that the raised lip which stops you selecting reverse instead of second was laying over to the right (as you look in from the back of the bus to the front).
So, put the bus in second gear, loosen off the bolts holding the shifter to the floor, then slide it all the way to the left (kerbside) and tighten them all the way home. Then, if you look at the side of the shifter baseplate from the drivers side you will see a small dimple in the metal, drill a hole just above this dimple, I did a 6mm hole, and then use a countersink punch (for recessing nails into floors, etc), and move the raised lip over to the kerbside about 5mm. Keep trying the selection and adjust the lip over until you have it selecting second gear only until the trigger is lifted to select reverse. It took me about 20 minutes and now it shifts real sweet. I did take a couple of pics as I was doing it but don't have them to hand, can forward if you need me to. It's a simple job and makes a huge difference.