Why is the internal resistance so much higher than the rest of the range?

Plate area. At 2.3 amp hours there is very little of it, and resistance rises as the block gets smaller: 32.10 milliohms here against 9.00 on the HRL1280W and 7.30 on the HRL12150WG. That is normal and expected rather than a fault. What it means in practice is that conductance readings on this size start from a high baseline, so a maintenance program that uses one threshold across a mixed bank will flag these long before they are worn out. Set the threshold per part number.

Is the FR case really standard, or is that an upgrade?

Standard on this part. CSB's price list lists it as HRL 1210WF2FR with the flame retardant case marked Standard rather than Available, which is why the FR is baked into the part number itself. That is not the case across the whole small band; the HRL634WF2 and the HRL1280W both carry FR as a surcharged option. Where a submittal has to cite the container rating, this is the easy one.

What happens if I charge it from a larger unit's charger?

You risk cooking it. Maximum charge current here is 1.0 amp, which is among the lowest CSB publishes, and a charger built for a 7 or 9 amp hour block will push several times that into a recovering cell. The float window is the same 13.5 to 13.8 volts as the rest of the 12 volt range, so voltage is rarely the problem; current is. On a multi-block panel the panel's own charger is sized correctly and this only comes up on bench charging.

How long does it hold up in a panel?

CSB rates it to eight years of standby service at 25°C and 10 to 12 years on the Eurobat 20°C basis, which is the Long Life plate doing its job. That figure assumes it stays near 25°C and stays on float. Heat is what shortens it, and a panel mounted above a ceiling or beside a transformer runs warmer than the room does. Where the environment is hot, plan on replacing sooner than the number suggests and say so in the maintenance schedule.