Maybe I didn't understand the whole point of this thread, but I'm not sure herald's spell slots/exertion is unbalanced:
- Adepts are the class with the highest amount of exertion per rest at any level after 6 (and they are on par with all other "standard" martials before that)
- Fighters do get slightly less than an adept, but more than all other standard martials, and from level 3 they get to use some maneuvers for a reduced cost, further boosting their effective exertion per rest. As they increase in levels, they can use more maneuvers at a reduced cost, and at level 20 they get to choose maneuver up to degree 3 to use for no cost, which gives them virtually unlimited exertion.
- Rangers at level 9 get to use one maneuver for free out of a menu of 2. Very narrow, but also means virtually unlimited exertion expenditure.
- All others have a standard progression of 2x PB.
Now, heralds do get a ton of exertion if they convert all their spellslots into exertion. However I don't think this to be an issue, because spell slots are recovered on a long rest. If we consider a standard day with 3 encounters, and divide the exertion evenly across encounters, we find that at low levels the effective available exertion per encounter is lower than any other martial class. This then picks up and becomes higher than an adept's, but only at level 17. At that point they'd have so much exertion that it won't probably even be possible to spend it on an encounter. Also, the steep acceleration in gaining exertion comes at the cost of level 3, 4 and 5 slots. The question of rough power equivalence between maneuvers and spells becomes poignant here.
So the issue eventually is one of too steep scaling (too weak at low levels and too strong at high levels). Also while it's true that a 1 level fighter dip is very good fir a herald, it's probably even better for a wizard (they get significant HP boost, proficiency in all weapons and armors, exertion, combat traditions and maneuvers, a fighting stile and a soldiering knack).
So maybe a solution to reduce this steep scaling issue is to give heralds the normal exertion pool, and give them the ability to burn spell slots to get more exertion, but at a higher cost. But at that point I'm not even sure there would be any need.