Why do three different sizes share an 800 amp discharge limit?

Because the limit lives at the terminal. The XTV12800, XTV121000 and XTV121100 all carry I2 alloy inserts on M6 bolts and all publish 800 amps for 5 seconds. The plate groups differ, which is why the short circuit figures climb from 1,917 amps here to 2,258 and 2,648 above, and why internal resistance falls from 4.70 milliohms to 4.10 and 3.60. Use 800 amps to check inrush on the load. Use the short circuit figure to size the protection. Do not use either to compare battery capacity.

How long will this hold a 48 volt load?

The datasheet answers in amps and watts per battery over time. At 1.67 volts per cell this block sustains 54.2 amps for one hour, 14.2 amps for five hours, and 7.75 amps for ten. A 48 volt string of four delivers those same currents at four times the voltage, so a 650 watt load at 48 volts draws about 13.5 amps and sits close to the five hour line. Work from the current the load draws, not from the amp hour badge, and leave margin for the cold end of the range where capacity falls.

Is 24 amps of charge current a problem for a small cabinet charger?

The opposite. It is a ceiling, and most outdoor cabinet rectifiers will not approach it, so the battery accepts whatever the charger can give. The consequence is that recovery time after a deep discharge is set by the charger's output rather than by the battery. A 10 amp charger will take the better part of a day to bring an 80 Ah block back from a full discharge. Plan the recovery window around the charger's output rather than the datasheet's ceiling.

Why is the 8 hour rating 74.2 Ah when the badge says 80?

Because the badge is the 20 hour figure. Lead acid capacity depends on how fast you take it out: at the 20 hour rate this block delivers 80.0 Ah to 1.75 volts per cell, at the 8 hour rate 74.2 Ah, and at one hour it sustains 54.2 amps, which is about 54 Ah. A four hour outdoor hold sits between the last two. When a runtime calculation uses 80 Ah for a four hour discharge it overstates the result by roughly a fifth, and on a cabinet that has to make it to morning that fifth is the difference. Size from the discharge table at the rate the load draws.