It's impossible to tell from a distance. All I have to go by, are clues you have given. The dark gear oil is one of those clues. All little color change is normal but I can't see how much color change you have. Dark gear oil isn't good, it points to a drive that's running too hot. Maybe you spend a lot of time up on plane? Maybe you need drive showers? Maybe it's something else?
You ask if it would be obvious when you disassemble it. Yes it should be for somebody who has seen and repaired this problem a few times. We know what a new cup looks like vs one that's slipping than grabs vs one that won't engage anymore. You say that it still engages after it slams into gear. All that means, is that it's not completely glazed yet. Kind of like an engine that will rev up with no load on it. Not really valid to the problem because a V8 will rev to the moon on 6 cylinders.
I will tell you my advice if you brought you boat to the marina, you need at a minimum, gear set and cone clutch and all new seals. This is because I'm a MerCruiser master technician and I'm at a MerCruiser certified shop. This is how MerCruiser wants the problem fixed.
So now you ask, what would you do if it was your boat? Not really a valid question because I would most likely do one of two things. First would be, I would sell the B3 lowers, put on B1 lowers and put the bad drive on the port side so forward is now reverse. This is because I really don't feel B3 helps that much on a low horse power twin application and I would make enough off the swap to buy a new clutch and seal kit. The second is depending how bad the cup was glazed I would either put a new clutch and seals on or install a gear set that we have laying around from a insurance repair. Most bigger marinas have these because after a hard impact strike MerCruiser recommends that all gears get replaced. So if nobody asks for the old parts back, we use them on our own boats or they get sold on eBay. And that's the difference between you and me. If those gears explode and take out the case (probably a 5-10% chance of this happening) it's not that big of a deal for me but it might be a big deal for you. This is also why I'm against used parts like gear sets ----- the only person who knows why they are for sale is the seller, and sometimes it's not a good idea to fully trust the seller.
Bottom line is you need a new clutch and probably more. One of the clues you gave me was that you have forced this drive out of gear. Brut force is rarely better than finesse and may have tweaked or bent some or all of the shifter parts. That means all these parts should be inspected by somebody who knows what to look for.
Also you had asked about the brass barrel on the shift cable was crimped and won't turn. This is another clue to me that you haven't read the service manual. MerCruiser covers this --- if I remember right they even tell you what size drill bit to use when you drill out the areas that are staked. So if your set on doing this repair, please take a close look at the manual for your drives and maybe even the latest Bravo manual set. Did you know that the early Bravo manuals left out an important step that should be done when shimming a B3 lower. The instructors words "all the mechanics doing this job without coming to this class will do this wrong"
