So, I'm going to try and answer your questions and explain a couple things at the same time. You're an adult (I assume) so you'll have to do what you want with your own money, but I think it's important to have all the info. If you need me to clarify feel free to ask.
"I'd like to send money to a local merchant's bank account (Banco Galicia) from my Canadian bank account and I'm trying to do it with Western Union."
As a general rule, independent of country,
do not pay for things via Western Union, there is no way to get the money back once it's sent.
"After submitting the transfer, Western Union says that the receiver needs to be registered and they also say "For all transfers, receiver may receive less due to foreign taxes." and "For transfers to a bank account, receiver may also receive less due to fees charged by the receiver's bank."
This is boilerplate for WU across all countries, the transaction, if it does go through, will be exactly what is shown on your receipt, however,
using WU to pay for goods or services sold abroad/to foreigners is illegal in Argentina and a violation of WU's Argentina specific Terms and Conditions. That's a problem for the Merchant, not you though, but again, don't pay for goods or services via WU as I said previously.
"Apparently Western Union needs to call my receiver and verify their information before approving the transfer."
They won't call them,
they'll send them this form to fill out
As you can see, here is the part where doing this is explicitly prohibited by WU, as it is a violation of their own Ts&Cs, and of
Comunicación “A” 6770 1/9/2019 del Banco Central de la República Argentina of which Western Union, and their local operating agent, Pago Facil, do not want to be caught facilitating:
View attachment 8066
"I'm not sure if the transfer will be approved and if the receiver will be charged some tax on the transfer."
It probably will be approved, and they won't be charged a tax, as the merchant is going to lie and say you're sending them the money as familiar support from abroad (the only reason you can select/use WU for), and the process is more or less automated.
"Is there an easier way to send money to someone's bank account? Ideally, would like to do this all online so I don't have to carry cash with me."
This is why I'm confused: are you here in Argentina already or will be coming here?
If you are just send the money to yourself, I don't really understand why you'd want to send the money to their bank account unless you're not setting foot in the country. In that case, there is no easy way to, Interac EMTs exist here for domestic money transfers only, and people don't use PayPal for commercial reasons as retailers are legally required to turn dollars (Canadian or otherwise) in to pesos at the official exchange rate, and most banks charge an expensive commission + a bad spread (the difference between the buy and sell rate) so even if you do everything legally, to sell goods and services to foreigners you are punished financially for doing so.
The reason they're suggesting you use WU is because they'll receive $165.30 per CAD instead of $83.14 (the official exchange rate) and they won't have to pay any commissions to their bank or taxes, but as I said, this is a bad idea besides being illegal for the merchant to do because you don't have any recourse in case they don't provide you with the good(s) or service(s).
Again, if you explain to us why you're choosing to pay this way we can provide you with some advise accordingly.