RPMH15-1.5

The datasheets specify the maximum capacitive load. If the combined capacitive load is higher, the converter may go into short circuit protection on power-up.
For switching regulators, but the output capacitor may discharge back into the output of the converter if the input supply is suddenly removed and damage the converter. Fitting protection diodes can avoid this reverse current flow.

No. Switching regulators function differently than linear regulators and this “trick” does not work. They need a very good ground connection to function properly.

All of our DC/DC converters contain a built-in input capacitor filter, so an external capacitor is not required for normal operation, unless specified in the datasheet. An input capacitor may also be required to meet surge requirements or to smooth the DC supply at the point of load. If several DC/DCs are powered from the same rail, then input capacitors placed close to the input pins are recommended.

No external components are needed. An input capacitor is recommended only if the input voltage exceeds 26V. An output capacitor helps reduce output ripple further, but the ripple is relatively low anyway.

The Innoline series all use intelligent controllers that measure the output current on each switching cycle (Current Mode Control). If the output is overloaded, the converter will deliver the over-current until either the converter overheats and shuts itself down (thermal protection) or the load current exceeds the safe limits. If the output is short circuited, the controller shuts down the output drive circuitry. The output condition is continuously monitored and the converter automatically restarts.

Type is not critical. Actually, a lower quality, relatively high ESR capacitor on the input is actually an advantage as its internal resistance helps damp down any switch-on surge oscillations.
A combination of tantalum or electrolytic in parallel with an MLCC on the input or output combines the advantages of both types (high ESR to reduce ringing, low ESR to filter noise).

The R-78 costs more than a linear regulator because it is intelligent. It may look similar to a three-pin linear regulator, but it is far more efficient and inside is a controller chip that protects the converter against overload, over temperature and short circuits. This makes it very robust and hard-to-kill.
Even if the converter itself costs more, the savings that can be made in the primary power supply (because it needs less output current), assembly (because there is no fiddly heatsink, screw, nut and thermal paste to worry about) and inventory (one part rather than 7 parts with the linear regulator + heatsink + mounting + input and output capacitors) mean that the overall power supply cost can be lower with the R-78 than with the ""cheaper"" linear regulator.