OMC Stringer 400 Shift adjustment

Hello i have an omc stringer 400 from ±1984 with some issues. Have made a video.
https://www.youtube.com/watch?v=S35iFOgPEj8

I am a beginner mechanic and don't know where to start and fix this step by step. Read some of forums to take off the lower unit to check the shift shaft. But can't find something where to start step by step (beginner)
Issue 1 - Scratching lower gears (following by issue 2)
Issue 2 - Shift cable adjustment, after going in gear, 2 sec of scatching and then goes into gear. Forward throttle, forward conversionbox = reverse prop
and vice versa. So cables are connected opposite
Issue 3 - Sissing sound from Air filter / carburetor
Issue 4 - Yellow oil in the lower unit (mixed with lake water)

Did you have got the same or some of these issues? Any help would be welcome. Appreciate it!

Does it have the dual shift cables going into the gearbox? Get a manual and try adjusting the shift interuptor switches as well it sounds like it is not fully shifting into gear?

The shift adaptor is behind the engine. With the engine off have someone spin the prop while shifting gears. Find the point where it touches forward gear and make a mark on the cable actuator then do the same in reverse and split the difference and zero the actuator with the interruptor switch. Disconnect the control canle and put the handle in neutral and adjust the cable to fit. If there is slop in the control cable adjust it more on the forward side so it fully shifts into forward gear. Your problem is probably in the adaptor housing or a worn out shift control?

Yes the dual shift cables goes from into the gearbox. I can switch between forward and reverse but with scratching. But this little handle in IMG_5396 is loose, the other is in place. Whats the function of this?
http://www.mediaframes.nl/Renken OMC/IMG_5397.JPG
http://www.mediaframes.nl/Renken OMC/IMG_5396.JPG

I've looked at the adapter housing and shift throttle control and check the cables you said. Reverse is going over line at adaptor housing, in the end, like the image its in full gear. http://www.mediaframes.nl/Renken OMC/IMG_5398.jpg

Indeed, the cables are to long to go full in gear. Have to shorten it thanks!. Another thing that came up when i shift to forward, the adapter housing is also going forward but the prop is going reverse. And vice versa. Did the problem is in the gear shift (lower drive)? this shift unit: https://tinyurl.com/y9muy382

If yes how to take of the lower drive? did i take all oils out and where to start

Anybody? any help would be welcome

Do you hav a left spinning prop? The adaptor housing can be tricky to deal with a good OEM manual is recommended. You said it goes in reverse have you had it on the water? Does the boat go backwards in reverse? Is the pointer close to zero when the drive is in neutral?