Is your 15 amp shore power connection metered, so you are paying for every watt-hour you use?
If it's a flat-rate for the shore power, 15 amps into the Magnum is going to provide like 80+ amps of DC charging current, like you said in absorb mode. who cares about the solar output at that point. once you're back to 95% SOC and in float mode, the solar would start contributing. I think I'm missing something, doesn't sound like a problem to me???
Now, if your shore power connection is metered, use the ARC remote to switch the Magnum's charge control mode to SOC % or Time of Day ... this way the charger will only kick in if you're getting too low, or when it's dark out. That way you can maximize your use of free energy from the Sun.