how to repair leaking water main
Overall, your plan to run PEX inside the HDPE and connecting them is acceptable.

However, I would not try to jury-rig fittings together. It sounds like you are trying to find a fitting designed for PEX that just happens to "fit" the HDPE, or vice-versa. This is a recipe for mechanical failure. After putting in all this work, the last thing you will want to do is dig it up again for another repair.

Instead I would find a plumbing supply house (not a big box store) and ask them about the proper fittings that are rated for direct burial, or the right way to bury a proper fitting.

You should seriously consider running PEX the entire way. How far are you from the spring? Running some steel cable down the HDPE first and then pulling the PEX all the way through may be a better fix.