"Are you sure the Smartpass cannot pass 80A?"
I'm quite sure that it can.
I'm equally sure that your battery too could pass an 80amp Charge current, but it shouldn't. Ever. EVER.
Quite apart from the fact that your battery's maximum charge rate is probably about half that, consider the Voltage.
The Maximum charge voltage that your battery should be subjected to is specified by the manufacturer as 14.9 volts. Ask yourself this. What would the charge voltage have to be to pass a charge current of 80 amps? I cannot answer this, but I can assure you of one thing. It will be way, way more than the specified maximum of 14.9 volts. I once fitted a home-brew manually variable alternator controller to my boat. I could crank the charge current up to 80 amps, no trouble. The problem of course was that to do so, the charge voltage had to be set to over 20 volts! Boiling the batteries and wrecking on-board electronics would be certainties.
"Do DC-DC converters pass that much from the alternator, or the maximum the alternator can produce?"
Neither. As I say, it is not going to happen (or shouldn't ever happen). They won't take that much from the alternator because that is too much for the battery and the controller should protect the battery from such overcharging. The maximum alternator current is also irrelevant. While it may be a 200 or 300 amp alternator putting that into a battery will quickly destroy it. Again, the controller is there to regulate the charge and protect the battery from overcharging.
"As mentioned previously, I can see alternator charging required if you park up for a day or two and there is no sun."
Some people are surprised to find that solar panels do not need sun. They run on light, not heat.
In fact, solar panel output is higher on days of "bright overcast" than full sun.
"....and your batteries are 50%."
After just a day or two? That would be evidence of insufficient solar power or insufficient battery capacity.
Or both. With 300 watts of solar power and 200Ah House batteries, we can count the number of times we have been as low as 50% on the fingers of one hand - and that's in over 8 years of use. While it is true that we move on almost every day, we do not have to use any engine charging at all, except in extreme conditions that we have experienced only once (deep South in the Winter) and that's never going to happen again!
Mike, the experience of MarkCraven is of little relevance to you and your situation.
(1) His battery capacity is triple what you are considering.
(2) He is using AGM batteries which can take a higher charge current than your (proposed) AGM/Gel battery.
(3) He does not say what charge Voltage he is subjecting his batteries to.
(4) He could have three 12 volt batteries in parallel and the 80 amps that he sees would translate to a quite acceptable 27 amps per battery.
(5) He could be overcharging his batteries, resulting in their reduced life expectancy.
(6) He could have voided his battery warranty by discharging his AGMs so deeply - and charging them at too high a rate as well.
Good luck with that 100amp charge rate Mike.
If you ever see it, you'll wish you hadn't!
If you really do want charge currents that high you should be buying a Lithium battery, not an AGM/Gel hybrid.