Grading is the first suggestion I make when trying to keep water out of a basement. Then you need to make sure that your gutters are not leaking and your downspouts are led far away from the foundation, where the water will move away from the house.
If you've already purchased this proeprty, or found that you purchased another home and have a wet basement, go outside in a heavy rain and watch the flow of water.
I've seem homeowners unknowingly take on their neighbor's water.
I can't comment on this home of course, but all the inspectors that I use say that proper grading and keeping gutters and downspouts clean and in working order is often all that's needed to keep a basement dry. Still, that's not to say that grading will fix this home.
Your problem is difficult on a couple of different levels. Ideally, you would like to have an inspector or water-proofing specialist inspect and give you some kind of warranty at purchase. Unfortunately, I don't know of any company that provides this. Also, if grading doesn't solve the problem, then it could be rather expensive to fix. And, the only way to test the fix is to see how the basement does during a long, hard rain. When's that going to be?
Here's what I would do.
#1. You definitely want a professional inspector's opinion on the basement. You are correct -- the inspector is very unlikely to give you a diagnosis with the certainty that you would like. And, he's definitely not going to give you any warranty. It's just the opinion of a professional, but you need it (you can always get other opinions as well)
#2. Make sure the seller documents the issues with the basement and all repairs made in the disclosure package that would be part of the contract. If later you discover that the seller knew that the issues were more extensive than documented, then you have a legal path to get damages.
#3. Contact your home insurance provider and get rates for this property. If the current owners have had several water damage claims, then *your* insurance company will know. Properties with extensive claims become stigmatized (don't know if that's the right word) and are more expensive to insure based on past claims.
#4. Lastly, if you want to move forward with a contract but are still unsatisfied that the basement is really dry, then you could request that escrow funds be held back from the seller proceeds of $10,000 to $15,000 or more (you'll need to get some quotes to help justify this figure) for 12 months . This money would be used to fix any water penetration in the basement in the first year. The balance would be returned to the seller if all/part is unused. The seller won't like this, but I don't know of any other way to give you some level of real financial protection.
This is a tricky problem with no easy answers. Your concern is on target - this is a risk that you need to mitigate before you make the purchase. I hope this helps - good luck.
If the cause of the problem was do to water not being directed away from the home through slopping, gutters, trenching etc. you may have solved the problem. However, there are many other sources for water intrusing that should be considered in order to find the comfort you are looking for.