Your idea is sound. I just designed the charging system for a 72' power catamaran
with a 7610 supplying the navigation
battery bank in the pilot house. Total wiring
run was 80' (100 amp alternator). I used 1/0 for about 7% voltage drop. Size your cabling for < 5% voltage drop and put a proper fuse where you connect to the battery and you will be fine.
By the way, AGM batteries are sensitive to high voltage and charging voltage should be decreased with increasing temperature from standard 25 C (negative temperature coefficient) and the charging voltage should be increased as the ambient temperature decreases from 25 C. Most AGMs use a value of 4 to 6 millivolts/C/cell or 24 to 36 millivolts/C for a 12 volt battery. What all this means is that to ensure long life for your AGMs, use a charging source with the correct charging profile for your AGMs and ensure that the charging source uses temperature compensation to adjust the charging voltage in all three stages.
Now for your specific questions:
Q2> A good rule
of thumb is to charge flooded batteries with a charging source of 15 to 20% of capacity. AGMs can easily handle a charging source of 40% with thin plate pure lead (TPPL) batteries able to absorb a charging source of 100% of capacity. In your system the charger is a bit undersized.
Q3> I don't recommend mixing charging sources. With different charging algorithms, they won't generally play nice with each other.
Q4> The engine
starter may use 250 amp x 5 seconds x 1 hr/3600 seconds = 0.35 amp-hrs (assuming a well tuned, 80 hp engine). With your alternator connected to the starting battery, it will take only a minute or two to charge your starting battery and for the 7610 to close to start charging your house battery. So far so good. The starting battery will be ready to go into float but the combined bank will look like a battery that needs to be bulk charged. In fact, the starting battery will share some of its charge with the house bank. As the alternator charges the combined system from bulk to absorbtion and then into float, the starting battery will end up being subjected to bulk and absorbtion voltage for a longer period of time than necessary. No current will flow into the starting battery after it has reached equilibrium. You cannot pack more electrons into the electro-chemical process than the electro-chemical process will allow at a given voltage and temperature.
The system described is not laboratory perfect but it is robust and will work well. I have designed and installed many systems in sail and power boats from 36' to 72' just as described and these systems perform very well.
I would recommend installing a Blue Sea 6006 switch in parallel with each of your 7610's. This switch will allow you to bypass the 7610 should the need arise.