Do you have any sort of pump or fountain going? These are often common spots for leaks to happen.
You know, you mentioned not having the time to do any of the work, and you also mentioned having to pour sand, and not being able to dig up the preform, etc. It sounds like you might be over thinking the difficulty of the work involved.
You’re going to have to move those rocks no matter what route you take. Once the pond is drained, there really shouldn’t be anything for you to have to “dig up” with regards to the preform. It might resist a little bit, depending on how long it’s been in place — but it should come out of its hole without too much effort. And you wouldn’t have to put sand in the hole at all — a protective underlayment to go under the liner is all you’ll need — and you need that anyway, when you use a liner.
Just MHO. It’s not that much work if you really want to use a liner, but it will pay in the long run to do it right the first time.