I have only been able to invest in 27 merchants in my game.

I was able to invest in the following NPC's stores:

• Nurelion/Quintus (White Phial)
  
• Oengul (smith in Windhelm)
  
• Wuunferth in Palace of the Kings
  
• Solaf (Gray Pine Goods)
  
• Beirand (Solitude smith)
  
• Endarie/Taarie (Radiant Raiment)
  
• Fihada (Solitude fletcher)
  
• Sayma (Bits and Pieces)
  
• Sybille (Blue Palace)
  
• Lami (Thaumaturgist's Hut)
  
• Frida (Mortar and Pestle)
  
• Rustleif (in Dawnstar)
  
• Bothela (Hag's Cure)
  
• Calcemo (Understone Keep)
  
• Lisbet (Arnleif and Sons)
  
• Balimund (Scorched Hammer)
  
• Bersi (Pawned Prawn)
  
• Elgrim/Hafjorg (Elgrim's Elixirs)
  
• Wylandriah (Mistveil Keep)
  
• Alvor (Riverwood smith)
  
• Lucan (Riverwood Trader)
  
• Ulfberth/Adrianne (Warmaiden's) - I was not able to invest in them separately as indicated on the UESP - possibly addressed in a patch?
  
• Arcadia (Arcadia's Cauldron)
  
• Belethor (General Goods)
  
• Elrindir (Drunken Huntsman)
  
• Farengar (Dragonsreach)
  
• Birna (Oddments in Winterhold)

I expected an investment option from the following, but it was not available:

• Revyn in Sadri's Used Wares
  
• Angeline/Vivienne in Angeline's Aromatics
  
• Ghorza (Markarth smith) - initial investment offer dialogue line appears, but the only follow-up dialogue option is to decline
  
• Moth (Understone Keep smith)
  
• none of the Khajiit (Ahkari, Ma'dran, or Ri'saad)

Feedback/confirmation/contradictions?

Thanks.

IIRC I was able to invest with the Moth, the male orc smith who works inside Understone Keep. He has a favor quest though and I've always completed it, I don't know if it's required before you can invest with him. Doesn't seem that it should be as the reward for completing it is something else entirely. I can confirm that I had the same experience as you with his sister who runs the forge outside, that's a known issue according to UESP the last time I checked. The initial dialogue option is there but if you select it there's no follow-up option letting you give her the money. I seem to remember there were a couple of other merchants who had actual shops (not stalls) that I wasn't able to invest in, seems like Angeline was one but I can't remember if Revyn was the other.

You can't invest with the Khajiit, the only way to get more money into the hands of the caravaneers is to do the Thieves Guild quests and get to the point where you are offered a special job to set up a deal with them. After that is completed the main merchants (the ones sitting in the tents) won't have more money but another member of each group will serve as a fence for the TG and usually have around 4000 gold on hand.

I was able to invest in both Adrienne and Ulfberth.

Adrianne/Ulfberth store situation was quite confusing for me. If you deal with Adrianne first outside the store, then go inside, Ulfberth has different money and inventory from Adrianne. But if you come back out to Adrianne, she has Ulfberth's money and inventory.

So now I make sure to finish dealing with Adrianne completely before going inside the store.

And if you see Adrianne inside the store after 6 pm, that means she's working with Ulfberth's money and inventory.

Yeah, did the quest for Moth (to get a daedra heart IIRC), but still no invest option. Figured the Ghorza thing would be confirmed. And thanks for validating the Khajiit deal - prolly should get fixed on the UESP, since they have the 3 cats listed as investable (if that's a word).

Shoot. See, I tried to talk to Adrianne outside the store early in the day (when she has her own inventory), but I didn't get the dialogue option. I'll give it another try, although it seems it may be too late. Thanks.