Sorry, a bit late to the party.
The 45Amp alternator is fine. The optima will not take very much current, as it is Lead and will never be discharged deeply. You might be getting close to the rated output, but the big issue is overheating the alternator. A 45A alternator has the same heat dissipation as a small case 70A. So while you need to de-rate the 70A, you need to de-rate the 45A much less.
The charger (Orion if that is what you choose) only needs to support Lifepo4. It will not be charging the optima battery. Connect everything such that the alternator only charges the Optima. Then connect the Orion, fed by the optima to charge the Lifepo4. The charger will have a way to detect that the engine is running, and will only charge when it is. But it will be wired to charge from the optima battery.
You might be able to use the Lifepo4 to start the engine in a pinch, as the Daly disconnect is much higher than 250A. But, you risk damaging it. A better idea (what I do on my boat) is to parallel the batteries with the switch, then wait. The Lifepo4 will charge the optima. It should only take a few minutes to get enough charge that you can switch back to just the optima, and start that way without using the Lifepo4 to start.
100W of solar is pretty small. I have close to 800W for a 300Ah battery, and it isn't enough to fully charge it in a day. But, I get 3 days out of 300Ah so it works.