"I need to do something, because the last time I wore it in the rain it was leaking like a sieve through shoulders and tops of the sleeves within 5 minutes. I'm also looking at the ReviveX stuff, since it worked great on my WM sleeping bag."
There are two things you need to understand:
1. The DWR coating is NOT a water proofing. CSF simply prevents water from sticking to the outside of the jacket.
2. It is the membrain that makes the jacket waterproof and breathable. The membrane is between you and the outer shell of the jacket (typically a PTFE (A special type Teflon plastic film) or Polyurethan plastic coating or a combination of both (Gortex) is.
If you are getting wet in 5 minutes it means the membrain is damaged meaning the fabric is no longer water proof. Applying ReviveX will restore or improve the DWR property of the outer fabric but it will NOT repair the membrane. Marmont has had problems with the membrane delaminating from the fabric. When that happens the membrane can be easily damaged. Your marmont jacket is probably damaged beyond repair and needs to be replaced.
"There's lots of conflicting info out there – everything from there being nothing you can do to somehow using a hot dryer or iron to spring things back to life and relaminate the coat. I'd love to find out what the real deal is."
The solution to problem of a failed DWR coating depends on what the actual problem is.
1. if fabric has been washed in the wrong detergent. Then the best thing to fix it is to wash it with the correct detergent. However rewashing is no guarantee that the contaminate will be removed. If the contaminaten can not be removed it may not be possible to restore the DWR.
2. When you wear the jacket friction between fabric and your pack and other items will slowly damage the DWR. Lite damage to the DWR due to friction can be repaired by heat from the dryer. But if there is a lot of damage due to friction The dryer may not be able to restore the coating.
3. If the loss of DWR is due to either of the two above items, or both your only choice emay be to apply a new DWR coating to the jacket. This may or may not work depending on how much contamination there ise and how much friction damage there is.
There is no single solution to the problem since there are multiple causes and multiple ways of fixing it. The conflicting information you have heard is in fact not in conflict. they are all possible fixes for the problem and in some cases none will work. you need to try all of the solutions and if none of those work then you need to replace the jacket.