[RPG] Acquiring non-class skills for which untrained checks aren’t allowed after character creation

I would say no, for several reasons:

Ray of Frost on the PFSRD.

1. Ray of Frost does not freeze things, by RAW. There are spells that specifically say that they cause certain elemental effects (for example, Fireball specifically says that it "sets fire to combustibles") but Ray of Frost is not one of them. While it might be able to cool water or make a very small amount of ice, I doubt that Ray of Frost would be able to freeze enough water fast enough to break a lock.

2. Actually making the water stay in the lock is non-trivial. If you have some way of keeping the water from spilling out of the keyholes, then this isn't as much of a problem, but I don't think much water will actually stay in the lock after you've poured it in.

3. From a balance perspective, allowing an easy way to break open nonmagical, traditional locks using two 0-level spells is kind of cheesy, and Knock is 2nd level. I might be comfortable with players using a 1st level spell creatively this way, and definitely if they can creatively use a 2nd level spell for this kind of effect, but I think that using 0-level spells for a 2nd level effect is pushing it. I realize that Knock does more than just open nonmagical locks, but there isn't anything below 2nd level that can open locks, so I feel like that's a good baseline for this effect.

4. I'm not sure this would actually work, at least not in a reasonable amount of time. While you could certainly make a lock stop working by freezing water in it, breaking the lock entirely requires much more pressure than ice can reasonably exert. I find it more likely that the iron or steel that most locks are made of would just force the ice out of the keyholes or into other open areas rather than break. In the related Mythbusters episode, it required liquid nitrogen (which is likely much colder than a 0-level spell), and it still needed an impact to break the lock.

5. This is much more situational, but some doors (especially doors designed for security) don't open just because the locking mechanism is broken. If a door has a deadbolt attached to the lock, then breaking the lock won't let you open the door anyway. And if this trick works in a particular world, then it's very likely that locksmiths will adapt to this technique, and make locks that stay locked after being broken.

The easiest way is to add or subtract a bonus to the roll. Let's say a character has d8 in lockpicking. I want the cells to be opened 50/50 of the time, so I set the difficulty at -1 for that character. The chest needs to be harder, so you can set that at a -2 or a -3.

You always make the adjustment to the roll, and not the target number. I find it much much easier to do it this way. All my players know that all difficulties (except melee) are a 4+. Some adjust that to a 5 or better, some just add up there dice and difficulties and see if it's 4+.

You can also adjust a scenario to the character fairly easily as well. This can really help in the case where a new character comes in to the middle of a game, and the rest of the group have high skill. If the new character has a d4 in Lockpicking, you still make it 50/50 with a +2 to the roll etc.

One other thing to remember to look at with any player character, or other wild card is that a d4 is actually a d6 (all wild cards roll their dice and a d6). So, even a d4 skill will hit 2/3 of the time, and explode 25% of the time. Very often the second roll of the d4 will put them over the target number. I find that d4's & d6's explode a lot, and that can make for interesting games :) I don't try to balance this, since this seems to follow the "beginners luck" trope pretty well.