There is no such thing as a send address in Bitcoin. I've heard Electrum has a feature with a name like this, but it's misleading. Bitcoin payments are often made to an address, but when you spend them, you don't encode any sort of address into the transaction---you just provide the information necessary to prove you controlled the private key that created the address that was paid in the previous transaction.

When you start Electrum for the first time, it generates a random 128-bit number (a number between 0 and 340282366920938463463374607431768211455) called a seed. Electrum then turns this number into the seed phrase you see. Each of the possible numbers has a different seed phrase. It's an assumption of cryptographers than no one currently has the ability to try even a significant fraction of all the possible numbers (or phrases) to guess your particular number.

a. No. There is a mathematical relationship between your public keys and your seed, but it's another assumption of cryptographers that nobody can determine this relationship without your private seed. (Specifically, they need a number derived from the seed called the chain code.) So making Bitcoin transactions does not reduce your security.

No. As stated in #1, there is no from address in Bitcoin transactions. Each of the transactions you create will reference one or more previous transactions where you received payment.

No. Bitcoin is a pseudonymous system. In order to receive bitcoins, you must have a public key. In order to spend bitcoins, you must prove you control the private key corresponding to that public key. There are ways to make it difficult for people to associate your Bitcoin transactions with your real-world identity, but (in my opinion) none of them are easy and reliable enough for Bitcoin novices like yourself. I suggest you only use Bitcoin for transactions that you wouldn't mind becoming public at a later time.