Comments on your points:
1) Synthetic lubricants do not penetrate any gaskets or seals. That is simply untrue. What happens in about 1% of cases on older engines is that leaks develop from an already faulty seal or gasket. Conventional oils can leave heavy deposits over time. If a gasket or seal fails and begins to leak, the conventional oil deposits often will seep in there and clog the faulty seal up, preventing a leak. Conventional oil can gum up your whole engine, but can stop leaks do to heavy deposits. This is how STP Oil Treatment and STP Radiator Treatment stop leaks. Not the best fix. The better fix is to address the problem rather than gum it up. So why do leaks sometimes develop with high end synthetic oils? Because they clean out your engine. As the oil sludge buildup is removed, the grime clogging a faulty gasket or seal is also removed. This is the cause of the leak finally showing itself. Synthetics DO NOT penetrate or degrade engine internals. If you develop a leak after switching to synthetics, which is very rare, you already had a problem with a seal or gasket, but it hadn't exposed itself yet due to sludge buildup. I recently picked up a 1993 Cummins 4x4 dually. Switched every fluid over to AMSOIL. That was almost a year ago and zero leaks. Actually, the underside of that '93 is cleaner than my '02 Cummins.
2) Synthetics not only provide great cold weather performance, but also hot weather performance and engine protection. Synthetics are more thermally stable at high temps. Think about the demands your oil-lubricated and oil-cooled turbo put on oil? Spinning at 30,000 rpm at 1,400 degrees EGT? That kind of abuse puts any oil to the test. Synthetics perform much better at those extremes with resistance to thermal breakdown and lower volatility at those temperatures.
3) As much as I tend to cringe when I hear of someone burning their waste oil through their injector pump and injectors, at least you are filtering it well with that Frantz before you put it in your fuel tank. That is a good idea. I assume you are doing this for lubricity? Engine oil, conventional or synthetic, is engineered to not burn well. It has a high flash point to make it as thermally stable as possible. Engine oil on average has a flash point much higher than diesel fuel. This can cause a poor or incomplete burn of the lube oil component of your fuel, which can lead to clogged injectors. If you want to add an oil to your fuel for lubricity (which I'll just point out is against manufacturer recommendations - covering my rear there

I would say add 2 stroke oil to your fuel. It has a flash point much closer to that of diesel fuel and is engineered to burn clean and completely.
-Chuck