Just tried it, I can write an email from one of my addresses: something@example.pro, other@example.pro, another@example.pro.
But not from an alias: something+johnny@example.pro, other+spoon@example.pro, another+kettle@example.pro.
Addresses are limited in number, 5 on my plan. Aliases are unlimited.
For the second, they've already implemented a feature on their web client to download emails locally so you can search in the body of the messages.
See: https://proton.me/support/search-message-content#the-index-o...