Second, no, Chris and I aren't "big Morel cheerleaders". We sell them, we endorse them, we put our name behind them, because they're the best lifter on the market. Doesn't matter who you bought them from.
Third, I never said valve float.
You asked the question. I gave the answer. I'm sorry of you don't like the answer and you can dismiss it if you like, insult me and Chris if you like, but the answer is that the only thing that causes tie bars to break like that is an out of control valve train. We sell thousands of sets of lifters a year. Broken tie bars are VERY infrequent (like maybe 2-3 sets a year) and they all have that same size rivet. There is no load on the tie bar until the valve train is out of control. When you have an out of control valve train parts start breaking. Tie bars don't just start breaking for no reason and they don't run 450 hours without breaking if there was an existing problem from the get go. The damage may have started before you changed the valve springs. At 50 hours, depending on the springs, they may already need replacing. What do the tips of your valves look like?
As far as the engine never seeing 6000rpm...maybe, but I've been around boating WAY too long to know better. You get aired out at WOT even a couple times on a trip and I guarantee you the engine sees over 6000rpm. Hard for me to believe that never happened...but I'll take your word for it. There could be a harmonic at 5000rpm that's completely destructive to your valve train. Could be smooth as glass at 4500, and even 5500. Things like this you just don't know till problems start showing themself. I think the biggest factor, however, is just plain old time. 450 hours on a set of these lifters in that environment...I'd say you have nothing to complain about...not that you're complaining, I just think that's your answer.