My questions:
1.a How long does it take to verify the validity of a single transaction (not included in a block)? I guess some assumptions must be made (CPU, memory, number of inouts/outputs in a transaction)
It depends on the number of inputs and signature operations. A transaction could theoretically take several minutes to verify.
1.b Is there a tool available with which I can test and observe the verification time (of a single and/or multiple transactions)?
I'm not sure, but you can probably write one.
2. Is transaction validation of a single transaction (not included in a block) similar to transaction validation of a transaction in a block body?
Thanks.
Yes.