Can running a second battery in parallel harm a stock alternator?

Adding batteries won't harm your alternator. Pulling way more current than your alternator can supply WILL harm your alternator.

You can hook 50 batteries in parallel with a stock alternator if the batteries have previously been fully charged. The issue is when you run too much amplifier power with that stock alternator. Your batteries may buffer a lot of the voltage drop, but your alternator will constantly be loaded down and putting out its maximum output. Eventually, the alternator will fail.

No, I don't think it's worth getting a HO alternator for 1.5k.

If those lawn mower batteries are sealed (AGM), then yes you can use those. If you don't already own them, don't buy them for this purpose though.

Go to your local battery shop. Ask them if they have any used AGM batteries that load test good and are priced reasonably

 
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