to be sure the game saved your progression, before disconnected, just harvest one farm even if it have only 1 gold. It will trigger the save(upper right of the screen), wait until the save disappear, then quit.
the bloonstones price is kinda cheap if you compare this game to all other game of village. Moreover, it's easy to have bloonstones, at least enought for a tier 4 upgrade: doing map without health losse (+hardcore mode), daily event, boss, and Ct milestone event. But i confess that without the ct milestone event, it's harder to gather gold
Challenge don't unlock upgrades, look in the challenge menu when playing faceless, you'll see there is a buff instead of the first challenge. This buff is only applied when playing as faceless, but sometimes buff challenge can affect other races.
Trying the angel challenge 2, waiting for 500 000 mana, almost generate 200 000 mana, accidentally cast tax collection, abdicate in total frustation and doing it again XD
 "Realm Grinder"){kind=icon}
 "Bloons Monkey City"){kind=icon}
 "Burrito Bison: Launcha Libre"){kind=icon}
 "Siege"){kind=icon}
 "Trimps"){kind=icon}
 "The Perfect Tower"){kind=icon}
 "Necronator 2"){kind=icon}
 "Upgrade Complete 3"){kind=icon}
 "Swarm Simulator"){kind=icon}