Agent wants 2.5% signed before he'll send me addresses on his off-market rentals

Sellers can still contribute to your agent's fee. What changed is that they can no longer advertise that contribution through the MLS, so it's negotiated inside your purchase offer as a seller concession instead of being posted in advance. That's what your agent means by "usually covered," and it's a per-deal negotiation with no guarantee.

What protects you is one sentence in the buyer agreement: an offset clause. It should say that whatever the seller pays toward your agent's compensation reduces what you owe dollar for dollar, and that you only owe the difference. Without that sentence you can end up owing 2.5% while the seller also pays a concession. Ask for it in writing and read the number it caps out at.

On turnkey specifically, ask him one more question. Providers who sell tenanted properties often pay a referral fee or a co-broke to the agent who brings the buyer. If he's collecting that and also collecting 2.5% from you, you're funding both sides of his file. A straight question, in writing, works: on this property, who else is paying you and how much?

Twelve months and a whole market is a wide agreement for someone buying one or two houses. It's common to negotiate the term down to 60 or 90 days, or to limit it to specific properties he names in writing. If he'll only work on the wide version, that tells you something about how much of his income depends on the exclusivity rather than the deals.

I'd ask him to write it property by property. One agreement, one address, fee stated, offset language in it. If the turnkey list is any good he'll still get paid on the one you buy. The double-dip point is the one I'd chase hardest, because turnkey sellers advertise agent referral fees openly on their own sites, so it's not a hypothetical.