Currently, no. This is a limitation with how iMessage apps work. However, I'm currently working on a solution.
No. As of version 1.1.0, your friends will no longer need to install TinyPoll to vote. Installing the app allows you to create polls, and vote directly in the iMessage thread.
Yes, with a few exceptions. Voting will always be free. You can also create 1 free poll every 24 hrs with up to 3 options. You can subscribe for $1.99 USD / year to unlock unlimited polls and polls with up to 5 options.
No. The upgrade only unlocks features for creating polls. Once a poll is created, anyone it is shared with can vote for free. Voting will never cost money.
TinyPoll uses servers to keep track of your votes. Those servers cost money to run every month. So in order to make sure I can keep the service running forever I need to charge an ongoing fee.
Please see this article