I've been reading articles and watching videos for days. Need to ask for the big brains to chime in...

2007 e350 4M Wagon, 160k mi. <<. LOVE this car to the moon and back.

I had to have the back-end airbags and compressor replaced (left-rear bag gave out, I kept going because it was driveable and I was in the middle of nowhere).

I'm currently in a pretty rural area in Mexico where its 2.5 hours to the "only" MBZ mechanic I can find (right over the border in the US).

Mechanic had the car for three weeks to do the bags and replace the compressor and related wiring. Picked up the car, drove over the border and shut the car off in the inspection area and it wouldn't start 20 minutes later. Assumed it was battery related (after sitting for so long with the mechanic) and replaced the battery. Everything was good for about 4 days, then the car stopped cranking again. Lights come on (Voltage at 12.4-5) but car won't crank.

I went through all the fuses just to make sure all the fuses were good, but in particular for starter and ignition. All fuses were good.

What I've found though, is that if I go to the fuse box in the engine compartment, and unseat a few of the relays and then put them back, the car starts. It drives fine (voltage at 14.4-5 while driving, so alternator seems to be working), but when I park it for more than 15 minutes, it won't start again and I have to repeat the process (voltage when sitting is 12.4-5). Sometimes it takes a little bit to work after unseating the relays, and sometimes I have to do it a couple of times, with different relays - but it always eventually cranks.

I've been trying to figure out (with my limited electrical experience) whether this means a bad relay? Bad alternator (even though it seems to charge the system)? Bad starter?

Help. I'm going nuts and it would be a three-hour trek to get it back to the "Mercedes" mechanic. It's not that I couldn't find a mechanic somewhat nearby, but MBZ is NOT a common car in this area and I'm sure they would have no experience with this issue, so I'm a little hesitant to involve the local guys.

replace the starter and relay both common or just relay

I've ordered the starter and the relay both, just in case. I'll confirm when they are installed and car is working consistently again.