Foward and reverse lever stiff and wont go into gear [mariner 5 horsepower outboard]

what is stiff (add a noun here)
blush.....
clutch pedal stiff
shifter lever stiff.
grinds , a bad clutch will make the lever seem stiff or will grind, if forced.
gears balk? same as stiff.
false neutrals. (slips , gear not engaging)

go google and type in-- workshop manual for mercury optimax outboard-- there is apage mercury- marine.servicemanualvault.com
which sows manual in pdf form for $4.95 a down load
run through the pages to find the transmission section and if it suits what you want , down load it and have a manual for all the items you need to do
talk with a mercury service shop and get the details of how to do it
I'm sure that you have to remove the leg from the power head so that you can get the selection lever free from the forward/ reverse dog before that gears will come out of the case

check in the selector turret at the top of the box that the adjustment has not altered or gone stiff. Normally you will need a small amount of downward force on the gear lever to prevent un-required reverse selection during normal gear selection

if you have an automatic gear box , then it is very possible that you have reverse band is broken inside gear box , , light will come on because switch for reverse lights may be actuated by shift lever not gears .

there is a foward band and reverse band in auto gerabox's so if you are able to drive forward this gearbox will need to be repaired or replaced .

if manual gearbox , you may find a reverse linkage loose , or gears dammaged in gear box .
good luck hope this helps

Based on the information you have supplied, the car is not shifting in to the correct position. So when you try starting the car , it will not allow the starter to crank thinking it is not in park (which it is not...in reverse) I suspect that the shifter cable from lever to the transmission has become stretched over time and use. It can be adjusted at the transmission to shift cable hook up. The connection at the lever base could have also get disconnected. Try that first

check shift cable to transmission, could be stretched out or broken! if broken it will stay in one gear,because there is no movement in tranny lever.for the gears,

With your lower unit off have someone shift the engine into forward while watching the gear shift select shaft's rotation,let gearshift remain in forward,now place your lower unit in forward and reinstall. In the future just put engine in forward when installing a water pump as there won't be a need to move gearshift

The torque converter has gone out.

how many miles on trans? does fluid smell burnt?350 turbos are good tranneys but they do go bad. in my experance its reverse that goes out first on thies transmissions but thats not saying that it does not happen the other way either. my best guess is you need a rebuild. when you say it tries to move in forward gear that tells me that the clutches are shot. 350 turbos are worth rebuilding. its up to you.

In order to disconnect the gearshift linkage from the gearshift operating handle, a safety clip was rotated about 90 degrees clockwise. Once in this position, if you pull the safety clip towards you, it comes completely out, which then permits the gearshift rod to seperate from the operating lever.