You need a basic lesson in Lead Acid Battery chemistry:

Terms:

Amps - the amount of current a battery can put out at a given voltage.
Volt - the energy behind those Amps above.
Amp-Hour - How much energy that battery can produce for one hour at a given voltage.
Self-Discharge - the amount of potential energy the battery loses while sitting inert.

Now, a battery is a chemical engine, in this case using lead and lead dioxide plates submerged in diluted sulphuric acid, separated into cells. The chemical reaction (exactly what that is not important) is limited by the surface area available for the reaction and the strength of the electrolyte (acid).

So, a 450AH battery is one that will deliver 450 amps for one hour before complete discharge. Such a battery will self-discharge at 22.5 AH per month.. For ease-of-math, let's use one (1) AH per day. That is 42 ma per hour. So a maintenance charger would have to make at least 42 ma per hour to overcome self-discharge.

Anything more than that will (theoretically) 'charge' the battery, assuming a (partially) discharged state initially. A fully charged battery subject to additional charging (overcharge) will 'sulphate' - break down the water in the electrolyte into Oxygen and Hydrogen, increasing the acid concentration. Actual voltages are greater than 2.1 V per cell in order to initiate and maintain the chemical reaction required to reverse discharge.

Meaning that charging a battery is a balancing act.

Going through your questions:
a) Absolute minimum charging current: That which will overcome self-discharge.
b & c) Some industrial batteries have the individual cell anodes and cathodes accessible for just that purpose. Otherwise, one takes the total divided by six (6) for a nominal 12V battery. The resting voltage for a lead-acid battery is very roughly 2.1 V per cell.

Temperature Charging Voltage/Cell Charging Voltage (12V) Gassing Voltages
20 °C 2.43 to 2.53 14.58 to 15.18 2.415 14.49

Temperatu Chemistry, remember. Reactions are affected by temperature.
Charging Voltage per Cell: What it takes to initiate and maintain the chemical reaction for charging. NOT equivalent to resting voltage.
Gassing Voltage: what will initiate electrolysis in a fully discharged battery, and cause some electrolysis during the charging process. WHAT THIS MEANS: why it is that lead-acid batteries need to be checked for electrolyte status water is lost during the charging process, always, to some degree.

Lead acid batteries are not complicated, but they are also prone to abuse and failure if the basic chemistry is not understood.

Peter Wieck
Melrose Park, PA