1. Spell-casting failures.... Another solution could be to only worry about the number if it is negative, and then reverse its +/- and add it to the spell failure roll. This is what I am currently doing.
That's how I would do it too: add up the total modifiers, if it is positive disregard it, if it is negative add it as a positive value (e.g. if there is a net -20 modifier, add +20 to the spell failure roll).
The same underlying rule applies in RMU except we made it more clear:
The roll for spell failure is an open-ended d100 roll. Modify the roll as follows: add up all the modifiers that applied to the Spell-casting Roll, sum these results, change the sign so that the number is positive (or zero if there was no overall negative modifier to the SCR), and add this number to the rolled number. 2. Demonic possession of Undead.
It's theoretically possible. That's part of how Black Reavers are created in fact.
However, if you're talking about the Demonic Possession spells on the Sorcerer's base list Soul Destruction, they are type Fm spells, which means they are mind-affecting. RMFRP says (pg 154) "It should also be noted that these spells are ineffective against any creature or entity that does not have a 'mind' per se (e.g., Undead, plants, politicians, etc.)."
3. Magical PP multipliers (and adders, I suppose).
RMFRP pg 80: "Only one bonus spell item {i.e. an adder or multiplier but not both} is usable by a given character between periods of rest (usually at least a three hour sleep period). In order to gain the benefits, a bonus spell item should be held or worn, and it must be with the character during the period of rest prior to its use." I would certainly end the benefit for one character if a different character then used the item during their own rest period.
Attunement is not actually required for adders and multipliers though, just as it is not for items that just give other kinds of bonuses.
4. Haste.
RMSS Spell Law (page 232) says:
A person under the effects of a Haste or Speed spell will have 200% activity each round, but must still abide by the turn sequence restrictions. The effects are summarized below.
Combat -- two melee or missile attacks may be made per round. All bleeding critical results applied to a Hasted individual are doubled, and rounds of stun, parry, or no parry are applied at 2 per round.
...
Spells -- Only one spell may be cast per round, no matter whether Hasted or not. In addition, all spells cast upon a Hasted individual last their normal duration.
Cumulative Haste/Speed -- Only one Haste or Speed spell may be in effect on an individual at any given time. In addition, as Speed requires a 50% activity penalty after the spell effects end, casting a Speed or Haste at that point will only bring a person to normal activyt, rather than extending the 200% activity time (50% x 2 = 100%).I assume Adrenal Speed is likewise not cumulative with Haste and Speed.
So, while hasted, you can make two attacks, but you are still limited to a total of 3 actions (one per phase). But, there is no limit on the amount of activity per phase -- those limits are only for movement (20% during snap, 50% during normal, 80% during deliberate, any remaining movement can happen after deliberate is resolved). If those limits applied to other actions, no one could attack during the snap or normal phase, since melee requires at least 60%.
Thus you can make two attacks, which must be during different phases, but you can pick which phase.
You could also make three attacks, with one in each phase, each as a 60% action (at -40). That's perfectly valid assuming you don't need any activity for movement. If your foe moves away during the round you are out of luck.
If you cast a Haste spell in the Snap phase, it seems ok to apply it to the current round, but if you cast it in the Deliberate phase you can't take any other actions (except movement). It would make more sense to give the benefit of the spell in the following round.
