"Can I convert a propane water heater to natural gas (or vise versa)?"
Due to possible liability, we recommend that you do not do so unless the manufacturer specifically has instructions that state that it is ok to do so.

"Do you think gas water heater timers are a good idea?"
No, we are not aware of any gas valve that is designed to constantly be turned off and on.

"How do I convert from 'Inches of water column' to PSI?"
1" of water column = 0.0360 PSI
and
1 PSI = 27.7612 inches of water column.

"If I set my water heater control at 130°. At what temperature does the valve actually come on at?"
Gas water heater controllers are designed to have a "hysteresis" effect in their start-stop temperatures. If you control valve is set at 130° the heater will probably not come on until the water drops to 115° or so, and then heats water until it rises to over 130°. The reason for this differential is efficiency - when a burner first starts it is not fully efficient, and it is desirable to minimize the start-stop cycling of the burner.

"I can't seem to get the water in my shower hot enough. It uses an anti-scald shower valve, but the water is coming out at about 80° - no-where near scalding. The gas valve is set high already. Do I need to replace the gas valve?"
The problem is most likely with the anti-scald shower valve. Anti-scald valves normally have two features - one of interest, and one most likely responsible for your problem. The one of interest is a pressure-balancing function for the shower-water delivery: once you have set the control to the desired position, any temporary drop in cold pressure will cause the valve to reduce the hot pressure in proportion. When the cold pressure has been restored, the valve raises the hot pressure in proportion, maintaining approximately the same hot-cold water mixture setting. In effect you set the mixing ratio by turning the knob, and the valve maintains the ratio by pressure balancing. You may notice pressure changes, but the temperature changes will be slight.
The second feature, and the one which is probably responsible for the cold water is an internal hot-limit setting for the valve. This is an internal adjustment "stop" for limiting hot flow at a preset high (hot) limit. What the installer is supposed to do is install the water system, let the water heater get to its maximum temperature, adjust the shower until the temperature is at a safe temp, and is then supposed to adjust this high-limit stop so the shower valve can be opened no hotter. In this way a shower-taker could never get scalded - even with the hottest water from the tank. It sounds like the high-limit on your anti-scald valve is not adjusted correctly.
How to adjust the high-limit is going to vary depending on what brand/model your valve is. Do to possibilities of scalding, we recommend having a local plumber make the adjustment for you.