What happens if you part ways with the agent, or they quit the field? Do you arrange payment directly from the publisher and then wrangle with them over errors? It seems as though the old agent would still have a stake in whether the statement is accurate, since their 15% depends on it, but can they still intercept the checks at that point?The way you describe this--"intercept checks"--makes me wonder if you suspect tomfoolery. I don't intercept checks from publishers. They're actually addressed to me, and there is a full time, highly competent bookkeeper here at New Leaf who makes sure you get your money in a timely fashion and correctly accounted. Every single agent in the world is REQUIRED to handle your money properly. If you suspect that is not happening, please email me for private guidance.
If you secure a new agent, are they generally willing to help look over statements for your prior books, or do they only deal with the ones they've negotiated? (My head is swimming with complications, especially all the different potential sources for subrights money).
Related thought, though perhaps this is a different topic: what happens when an agent dies? Can somebody 'inherit' their income, the way authors' rights can go to their estates?
To your more general question:
The agent who sold the book is, in theory, responsible for explaining the royalty statement and handling any problems. More often than not, the default agent who fields question about statements from previous sales will be your current agent. My authors are used to asking me first. Sometimes they'll preface it with "I'm not sure I should ask you" but my stance is: my client, ask me first.
If your agent parts ways with you via death, the commission proceeds are part of her estate. The publisher can split the money and pay the estate directly, and pay you directly. You can request the split without the agent's consent if you need to. (It's your money.) Generally most agents have plans for what happens when they kick the bucket. (Mine involve a large Viking funeral, professional mourners who wail and rend their garments, some sort of whisky bucket brigade led by Jeff Somers, and the lovely people at New Leaf seamlessly taking over my list.)