1.Finding a block means guessing a number.
Yes, true. In order for a miner to find a block he needs to do a trial and error method by inputting various nounce values and the resulting hash should be less than the current target. If the hash is higher than the target, he need to change the nounce and try again until he reaches the desired hash. Once he has found the hash he found the block.
2.That number involves showing that the transaction involves the right keys and then ..putting it through some meth equation? Is that right?
No. The transactions are struck in the mempool. These transactions are already validated by a bitcoin node and added to the pool of unconfirmed transactions. Once the miner finds a block, he picks up these transactions from the mempool (probably the tx with fees) and adds them to the block he is gonna mine. Along with a list of transactions included in the block, he adds a block header he has computed and a coinbase transaction which is the first transaction of any block that pays the miner.
What I wonder is ...is the miner constantly guessing numbers based on whatever transaction he intends to add to his block?
And so the transaction variable of the guessing is constantly changing?
I guess you have misread everything about how mining works and how transactions are validated! Miners constantly try to find a block by using up different nounce values. There is nothing to do with transactions and blocks.
So the miner is moment by moment is solving for a different pool of transactions?
Whoa, NO! Please read
Mining and Consensus : Mastering Bitcoin to get a complete insight of how Mining works.
EDIT : Oops, achow already answered...
Typing in mobile is one of the hardest jobs ![Tongue](https://bitcointalk.org/Smileys/default/tongue.gif)