You present a lot of valid points. Instead of taking the time to discuss each point individually, I think I can save time by quoting myself:
Players will not be able to create scrolls themselves, ever.
(Edit: ) We had this discussion months ago, and you haven't presented any new arguments. I'll summarize my objections below.
1) If scrolls were difficult and/or time-consuming enough to make, players would not be able to hoard them easily.
1a) Scroll-crafting can be made mutually exclusive with potion-crafting, item-crafting, and golem-crafting.
1b) If scrolls can be sold at the Magic Kingdom then the players themselves have a valid reason to not hoard unneeded scrolls, anyways.
Players should not have an easy way to augment cash for the side. Running out of cash should be a constant fear. Even if PC-made scrolls existed, they would not be able to be sold in the MK. MK Casters need to provide their own upkeep, and selling their services is primarily how they do so. They would strenuously resist aligned casters' attempts to get in on the action. MK casters have a monopoly on scroll sales, period.
2) Casting from a scroll requires an action, which is therefore not available for casting a spell the caster already knows.
2a) The scroll spell will likely be less powerful than the spells the caster knows, or if it isn't then it will be an expensive scroll.
2b) Spell scrolls can be limited so that they are always less powerful than the same spell cast by a caster who knows it.
2c) Specialized casters whom aren't always useful on the battlefield (like Dollamancers, Luckamancers, and Foolamancers) would use (and be given) scrolls more often than powerful casters whose spells are almost always needed in the field (like Healomancers, Shockamancers, and Croakamancers).
Specialized casters who want to also be useful on the battlefield can easily take the appropriate spells. Healamancers can take Hoboken now, Croakamancers can take Revitalize, etc. All casters get Fire. If casters want to specialize exclusively, they pay the price. Scrolls negate the penalty of specialization.
3) Scrolls, being single-charge items limited to casters, are not all that powerful.
3a) In fact, a level 4 Dollamancer can create the equivalent of a Hoboken Scroll that can be used by any unit for only 6 Juice.
3b) Outside the Healomancy and Shockamancy schools, few spells are desirable enough to be hoarded.
3bb) And Healomancers and Shockamancers are busy crafting potions.
If there are few spells that are worth being created as scrolls, then there is no need for scrolls. But thanks for pointing out the Dollamancer issue - the juice cost on that is clearly too low- it should be at least 8x casting cost, to bring it in line with potion creation costs.
4) Additional flexibility for casters isn't a bad thing.
4a) Trading a permanent and limited resource for short-term gain will mean that scrolls are never used frivolously (or if they are, the other players will shout at the one who did so).
4b) Scrolls would allow low-level and/or less-useful casters to fulfill a wider variety of roles.
4c) There currently aren't many sources of healing; so there's plenty of room for another.
4cc) Healing scrolls make perfect "use only in case of emergency" items.
4d) It would allow casters without lingering buffs to be more useful during down-time.
Casters can have as much flexibility as they want - they just have to buy the spells. No caster yet has chosen to sacrifice specialization for flexibility.
Healing should be rarer. Healers are valuable. If you want more healers, have casters take Revitalize.
If you want scrolls for emergency use, buy them from the Magic Kingdom.
Casters are already the 'favored children' of the campaign. Military units don't get nearly as many perks. Only a select few Military units have ANYTHING to do during downtime at all. While my goal is to make sure everyone has something to do, casters in general have far more to do than military units. They're each unique and delicate snowflakes, while many military units stand by the sidelines both between and DURING combats (*waves to the poor bodyguards and melee units*).
So, in conclusion, players will not be able to create scrolls themselves, ever.