Codes P0762 and P0700, transmission/reverse?

My vehicle listed above had had its transmission replaced. The guy who replaced it did not connect transmission lines near the radiator very well. Anyways after hitting a bump my check engine light came on and I started to lose power. I pulled over and called a tow truck. However, the guy I called decided to drive the car back 5 miles because by that time it had cooled down. He said there was a leak in drain pan and he repaired it. He lied. I went to pick up the vehicle after waiting for him to say it was good to go for 2 months. My shifter would not hit any gears. After going underneath and checking to make sure fluid was full, I saw the fluid was black and burnt. I knew I had leaked out all transmission fluid when I asked him to tow but fluid being black and burnt, I don't believe he even changed the drain pan. So, I had it actually towed to my house where it sat another 3 months and everyone who I talked to said I needed new transmission. After some googling I found out it may be possible to save it by doing a fluid and filter change, so I did it and viola! It worked. The car hit all gears and drove perfectly for about 3 days. Then I lost reverse. I have been without reverse for 6 months now. The code reader says p0762 and p0700. Shift solenoid problems. My car also wants to drive in neutral. If I park on a hill, I can put in neutral turn car off and roll backwards. I'm thinking I'm probably low on fluid since there is no dipstick and probably on uneven ground when I put fluid in. Plus, it took forever to get plug back in since I have to do it while running and that exhaust pipe is hot. Do you think it's possible I just need more fluid, or do I really need new transmission?