Lose binding screw. What next?

A little off subject but have to ask: How was the skiing/ snow conditions?

I absolutely agree with your belief that a thread lock compound can potentially cause an insert to spin itself out of the ski.

IF,.... you want to simply reinsert your insert in a fresh mix of epoxy, you should be fine. IF you are worried about your insert spinning out again, you can "stake" your insert with a single or double brad.  You buy some short headless brads, and a few small drill bits the same diameter of the brads. Reinstall the insert and let it cure.

Then, You drill the outside edge of the insert so you partially contact both the outer threads and the ski. Mix a tiny amount of epoxy, work it into the hole you drilled, (with a toothpick or the brad) then drive the brad in to displace the epoxy. Set the brad below the ski surface (so it doesn't rust) and make sure you get a drop of epoxy on top of it to keep moisture out. Your insert's threads now have a slot drilled down them and a metal brad in that slot that prevents your insert from turning in the ski...  

"Staking" the insert makes it so it will not spin out again by accident, and make a deliberate attempt to remove the insert difficult,... but not impossible...     HTH...

*Also,... if your binding isn't under pressure from the screws, then it's leveraging against your insert rather than the ski. The screws need to keep your binding and ski under pressure or eventually you will have an issue...

One of the things I like to do is paint a light coat of epoxy on my ski when I mount a binding, so my binding rests in a bed of hardened epoxy. This hardened negative image of your binding's base relieves much of the lateral force on the screws. IF you are worried about your binding sticking to this light coat of epoxy, apply some vaseline to your baseplate or maybe just some saran wrap when you bolt it up for the first time. Good luck.

Thanks, Marcus. That raises another question: For the backed out insert, do you simply re-glue? Or do you have to helicoil?

Frank's "brad-spike" advice sounds like a good option for pesky inserts. This time 'round, since I needed to ski the next day and the tapped threads for the insert were still in good shape, I just reglued and put the insert back in. It held fine for two light days of touring so far.

rlsg, North-facing aspects were great, everything else was mush. Skinable snow at maybe 6,500 feet (quickly retreating). We had hoped to summit, but things were so mushy that we weren't inclined to stay on the south-facing route any longer than we did. Ended up settling for a couple laps on the north-facing part of the northeast bowl, which was excellent.  

If I could do it again I'd approach from the opposite (northwest) side and try for the north bowl. Longer approach, but I bet the descent would be epic.

I'll try posting a trip report later, but in the meantime here's a link a pano I posted on Instagram, which will give you an idea of what the NE bowl looks like: (hopefully that works).

Conditions should be good for a while. McL is definitely in better shape than it was this time last year.

I've had issues with inserts backing out when I used the blue thread locker but the red stuff seems to be the right strength.

For the original "which screw is least important" question:
If you have to, the best answer is always to go without the one which is furthest from the pins. That agrees well with the "remove the 5th screw" answer if that's an option. For a four screw toepeice where all fasteners are equal distance from the pins (ie not a dynafit radical), it probably doesn't really matter which screw you choose. The joint will have a pretty similar stiffness regardless of which screw you are missing.

For inserts loosening our the ski:
I agree that redoing the epoxy job is likely sufficient. Its even better if you do an acetone cleaning first to get rid of any oil contaminating the bond. Most inserts will still have cutting lubricant residue inside and out when you get them. If in doubt, the brad-stake previously mentioned is a really, really good mechanical locking solution.

For the thread locker issues:
Blue is probably more appropriate for most people since it is has the convenience of not having to heat the fasteners to easily remove the binding. Red is tenacious, so if dealt with improperly it can make it really easy to strip screw heads. Otherwise red is fine from a practical sense. Here are a few ideas which might help the loctite be a more effective tool...
1) Failed thread locker is often a trickle down problem caused by not having enough installation preload on your screws to start with. For joint resilience, overtightened is always better than undertightened... until you shear off the screw head at least.
2) If your loctite is cured, tightening the screw again is breaking your threadlock bond.
3) Your fasteners might be oil/grease contaminated so the loctite never adhered fully. Try to clean both sides with alcohol, or acetone if you ever use silicone spray on your skis.
4) Your loctite might be expired and not fully curing.

I hope some of that helps.