I have an ally that is exceptionally exhausted. If I polymorph him into another form, do the negative effects of exhaustion—such as reduced speed—apply to the new polymorphed form?
No, the polymorphed creature can only use actions that a normal creature with the new form would be able to do. It comes directly from the definition of “game statistics”. The Statistics section of the The Monsters Manual starts with:
A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster.
The subsections include not only static numbers “Ability Scores” and “Speed” but also “Actions” meaning that RAW actions are also stats (and I cannot name a reason why it wouldn't be RAI as well). Under “Actions” it is written:
When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action, as described in the Player's Handbook.
Thus the polymorphed character's actions are limited to those the creature would be able to do were it a normal creature.
As for the possible inconsistency with the concentration ("A Wizard can polymorph self, therefore polymorph doesn't break concentration. If concentration is a class feature and it survives polymorph, all other class features can be used too.") there are two choices here:
Concentration is not something everyone has automatically. E.g. if a creature would not be normally able to cast spells it cannot concentrate on them either. In this case there is no consistent RAW answer: "statistics" cover all the aspects of creature -> if the creature cannot concentrate, polymorph breaks concentration -> polymorph cannot be used at self, even though PHB clearly states that it was intended to be used this way. Also Sage Advice says that concentration is not broken by polymorph, so this is definitely not a path to go with.
You can’t cast spells while you’re transformed by polymorph, but nothing in the spell prevents you from concentrating on a spell that you previously cast before being transformed.
Concentration is something everyone can do and/or let's use common sense. The point of the offensive use of Polymorph is to render a creature useless in battle, i.e. if a creature is turned into a sheep it is as harmless as one. If the class features were preserved we would have sheep going into rage (barbarian), teleporting (shadow monk) or... turning into dinosaurs (unless druids are considered to be shapechangers and thus are automatically saved) which is bizarre. Therefore, all the class/race features are gone. As for the possible inconsistency with concentration, it comes from a simple fact that designers cannot see all the connections between all the bits of the rules and sometimes we can only rely on how we think things are supposed to work.
The Petrified condition (PHB p.291)
A petrified creature is transformed, along with any nonmagical object it is wearing or carrying, into a solid inanimate substance (usually stone).
Polymorph spell (PHB p. 266)
This spell transforms a creature that you can see within range into a new form.
The transformation lasts for the duration, or until the target drops to 0 hit points or dies.
The target’s gear melds into the new form.
The Polymorph has a specific list of conditions that ends the transformation it forces upon it's target. Applying the petrified condition to a target effected by Polymorph does not meet any of these conditions, either directly or indirectly, through it being a second transformation effect.
Even the "non-magical object it is wearing or carrying" clause in the petrification does not apply to any gear as it "melds into the new form" and is therefore not being "carried or worn".
So the simplest and easiest to run and understand option you list is the one that occurs:
- The creature is still in form B and is still petrified.
(evidence of some good game design going on)
If the petrification condition ends before the Polymorph spell ends then the target continues to be in the form dictated by the Polymorph spell.
If the Polypmorph effect ends before the Petrification condition ends, then the Polymorph transformation clearly ceases. However the petrification transformation effect does not. This can justifiably be interpreted in two ways by the DM:
- the petrified target changes back into it's original form while remaining petrified. I.e. the petrification transformation effect is strictly "into stone", it changes a "flesh and blood" form B into a "stone" form B, so when the effect causing the target to be in form B ends, it changes into a "stone" form A.
- the petrified target remains petrified in the second form until the petrification condition ends. I.e. the petrification transformation includes the form of the target, it changes a "flesh and blood form B" into a "stone form B" and so while it is in effect it remains a "stone form B".
I don't know of any RAW guidance to choose between these, but my own call would be 1. as it has the potential to be the most dramatic.