Trainer bonuses apply to all units in the army, and they stack (which is why Henchmen can be so valuable - they can provide Trainer bonuses but they don't reduce experience like regular champions or Sov).  Potential trait bonuses apply just to the trait owner.

Guild Grocer is a variation of the new "Tough" ability you choose for your faction. AFAIK, you can have a global bonus of 30% maximum across your entire empire (so Tough and two Guild Grocers, or three Guild Grocers).  The +10% bonus applies to every unit in your empire, trained, Champion and Sov.  Individual bonuses (such as finite number of extra hitpoints, like a certain Fortress level bonus) still apply I think.  It's just that you can only have 3 stackable 10% HP bonuses at once.

What do you mean that it stacks on a per-city basis?  Do you mean that there is not a limit?  I guess the mechanics might have changed then.

Okay, well I guess the comment about there being a limit of 30% was erroneous.  Unless it existed for some versions and not for others.  Though I never really leveled up many towns in beta 4 and 5.

I think that base experience is divided up by number of champions for a given amount of XP.  I think creatures and troops only get a 1/2 of the base experience of the Champions.  I think troops get the same as with 1 Champion if there are no Champions.  Henchmen level up like Champions, and get XP based on base champion experience (and not troop experience) but they don't reduce the base champion experience at all like other Sovs/Champions do.

You play for 100 hours observing the experience gained from each battle...

Sorry to sound harsh , there are no easy ways to observe this, and it have taken me and some other beta testers hours to figure out the odds and ends of the current experience system.
We have several times asked that experience bonus attribute should be stated in the character details, I hope this gets in there along with critical damage multiplier.

That said, MOST bonuses stack, so its usually safer to assume it stacks than not. I don't think the Summoner bonuses stack though (several heroes with the "Summoner" trait), then again, just ignore those as they are rubbish anyways

Edit: Just wanted to state: I know for certain experience bonuses stack, 4 units with trainer 1 in the same group will increase XP gained by 40% to the whole group, if they are all heroes you will also experience the terribly hidden mechanic of XP Reduction. (divide amount of xp gained with amount of heroes in the stack).
Multiple grocers stack too, I don't quite like that feature though, this makes grocers extremely powerful in large empires, but that is personal preferences

Sincerely
~ Kongdej

I do better with math than drawn out explanations.  So if I am understanding this correctly then in a given situation where you would be awarded 100 xp:

Champions:   

A lone champion gets 100xp.

2 Champions get 50 each or is the 100 reduced to 50 then divided so each gets 25?

Soldiers:

A lone soldier gets 100xp.

4 Soldiers get 25 each.

Depending on the answers above I can then figure out what should happen when champions and regular soldiers are combined and produce some tests to validate.

Champions get the full XP divided by # of champs in a group.

Non-champion units always gets 50% XP each.

For example you have an army with 3 champions and 3 non-champions and they win a battle which yields 100 XP.

Each champion gets 33 XP.

Each non-champion gets 50 XP.

Then, XP bonuses get applied on a per-unit basis.

No, the Non-Champions would get 16XP (or 17 depending on how it gets rounded). Their XP is always based on how much the Champions get.

Henchmen are treated as Non-Champions for XP-purposes.

Geez, and no one referenced the stupid guide I've been putting together?  It has that information in the second 'concept', Experience.

Haven't noticed it, haven't been paying too much attention to FE forums lately.

Sincerely
~ Kongdej

OK, thanks everyone for answering the question. I note there is still some disagreement about what actually happens, but I think I get the gist of it.

Can I just inject at this point that it makes no sense at all?

Why would troops fighting with two or more heroes be less experienced than troops fighting with one? 

If the intention is, as I understand it, that there needs to be a mechanism to stop you just fighting with all heroes and not troops, then a more rational one should be found.