Water – Should anode rod avoid touching bottom of Water Heater

No matter what, it won't explode. Maybe a little bit of splat. But no boom. Ok, really, more of a squishing sound, the sound of your feet splashing through the puddle created when it fails. :)

There may well be an anode, connected to the input line. Or at least there was one, probably corroded away now.

Assuming a little water on the floor won't hurt anything, I would buy an inexpensive alarm that senses the presence of water on the floor. Its just a pair of contacts that set off a battery powered alarm when water bridges the gap. When it goes off, buy a new heater.

If some water in that area will be a problem, then the heater should be in a low pan that will catch a leak. If not, then consider a preemptive replacement, as this heater probably has only a few more years in it anyway.

I believe you are over estimating how much hot water is needed to fill the tub. It will be filled below the overflow drain that should be built into the tub. You can reduce the total by the size of the person getting into the tub. And most importantly you can reduce the amount of hot water by any cold water you include to regulate the temperature, perhaps you are only using 2/3 hot water to 1/3 cold water.

The important question you'll want to determine is how quickly the water heater can regenerate a full 50 gallons of hot water because you may want to heat the bath back up after a while, or use hot water for some other purpose after taking a bath. If you find yourself near the threshold of available hot water, you may find it easier and cheaper to increase the temperature of the hot water in the existing tank, rather than adding a tankless water heater.