T1 Terry wrote: Can the regen be limited to the max capacity of the motor or controller?
Yes, the controller will do that. For a controller like the Tritium Wavesculptor, you just ask it for negative torque and give it a current limit (all in one CAN packet).
For something like a Curtis 1238, you can have a regen pot as well as an accelerator pot, so you can hook up the regen pot to the brake pedal, or to some sort of dashboard or on-floor control. You can also limit the amount of regen that "full on" on the pot requests.
Is this regen operator variable between zero and full to the max limit set via a hand control?
Yes, hand control if you want, or pedal, or anything.
I'm thinking the limit would really be the max acceptance level of the battery pack
Yes, that's the most important thing. There is also passenger comfort; some passengers don't appreciate nose-against-the-glass regen
Whether a single large capacity cell pack or a number of parallel smaller capacity cell battery packs is the better option I'm still trying to decide.
Yes, but that's independent of regen considerations. If you have say 3 60 Ah cells in parallel or one 180 Ah cell, they'll both act much the same; if some [ in the parallel set of 3] want more charge than others, they'll shunt current between themselves, but their voltage will be the same (assuming sensible wiring). So from the outside, the 3x60 Ah and the 1x180 Ah look the same. As you say, smaller cells are easier to pack, but there are more connections to make, and the extra plastic and battery cage (of the 3x60) makes the pack slightly heavier.
AC or DC?
Actually, it sounds like you really want regen for your application; if that's the case, most would agree that AC is the easiest way to go. You can get regen with DC, usually with sepex motors, but that limits your choice of motor and controller. (Well, at the moment, the choices of AC controller are quite limited too.) What you probably can't do is take your choice of the many series DC motors, and expect to painlessly get regen from them. See for example DC Regen from practical and personal experience.
Edit: make the 3x60 vs 1x180 Ah comparison clearer.
Nissan Leaf 2012 with new battery May 2019.
5650 W solar, 2xPIP-4048MS inverters, 16 kWh battery.
1.4 kW solar with 1.2 kW Latronics inverter and FIT.
160 W solar, 2.5 kWh 24 V battery for lights.
Patching PIP-4048/5048 inverter-chargers.