This not the full game release yet. There are a couple more big releases before then. I believe as time goes by the game will be fine tuned especially for newbies that mainly use ingame cards.
The achievements are also work in progress, they were first released for the latest Rewards Season a few weeks back.

I use mostly blockchain cards so I can get to finish the game no problem. However there are some free cards you get in game that are as powerful as the blockchain ones. It's mainly a question to be lucky and summon them.

The timer/stamina is pretty standard for games like SoG and other games also use it. There's been discussion about this on Slack and the team has taken notice.
I don't think the timer will disappear but it will be tuned. The reason for the timer is that if you could play forever without limit you will get bored of the game pretty quickly.
This way you can spend some time playing the game then do something else and get back when the stamina is loaded up. I personally don't have a problem with the stamina.

For the earth card "daily quest" you need to finish one game in one of the 5 levels on the map or finish the whole infinity mode (35 levels). Each level you complete should count as one and of course make sure you have a earth card in your deck.

I'm looking forward to the mobile soft launch to check out what new features and improvements will be added.