the situation described is this:
Magician buys 20 ranks in Light Law, gets it auto, can then buy ranks and roll for fire law.
If, OTOH the magician buys 10 ranks in light law, and rolls, pass or fail they are done for this level.
If that magician gets another level, they start off with 10 ranks in light law SLA already. . .so they have a choice. . .they can:
Roll on light law, done for the level pass or fail.
Buy 10 more ranks to 20, auto get light law, then buy some ranks in fire law and proceed.
or
Buy ranks in fire law. . .loosing the 10 "Carryover" ranks in light law.
you only get one SLA roll per level, and you roll AFTER you have purchased all ranks, so no chance of any of those schenanigans you mentioned.
i.e. the process is:
1) Go up a level
2) Make Stat gain/loss rolls
3) Calculate DP based on your temp stats after step 2
4) Purchase ranks in skills with the DP total from step 3
5) Calculate the bonuses given based on new ranks.
6) Make Spell Gain Roll (SGR) if you either purchased ranks in Spell List Aquisition(SLA) this level, or carried over ranks of SLA from a failed SGR from last level.
Since by the time you reach step 6, you've already spent all of your DP in step 4, there's no way to roll, THEN get more ranks, until you go up a level again.
That help?