The best suggestion is to gradually adjust the outgoing melody (120 BPM) to half the difference between the two or 125 BPM, using the manual tone attenuator, and then press “sync” on the 130 BPM melody in the other deck (which means it will slow down to 125, which is better than 120). Once you've done the mix, you choose your next song and you also do this “50%” trick with the new melodies. In this case, you've reduced the BPM jump to 5%, which is much better than the original 10%. While the downside here is that any adjustment you make will probably be less precise if set to one of these settings, the good thing about (especially) a 100% option is that you can use it as an effect, similar to turning off the battery of a deck of discs when it's playing, so that it will slow down gradually but audibly.
Just set your tone attenuator to 100% and slowly move it “down” (which is actually up or away from you) while playing a song. Of course, you can speed up a track by doing the same. The problem is that locking the keys can make melodies sound bad: thin, rough and, in general, a bad substitute for the same track that plays without the key lock turned on. It's a symptom of your computer doing its tricks to keep that musical key where it should be.
It doesn't always sound bad, and not so bad when the track is close enough to its original BPM (see point 1 above), but it can still be a problem. When combined intelligently with the skill taught in point 1 and you always have to keep an eye on the BPM to keep the variances as low as possible, this is one of those tricks that can fool most people, most of the time; it means that, for most of the time you spend as a DJ, sound quality improves markedly, while you can take advantage of harmonic mixing techniques. I don't think the pitch fader is dead, far from it. Although it's not present in some DJ controllers (I think the Reloop Contour and the DJ Tech 4Mix) and has been transformed into a tone knob on at least one other (Novation Twitch), it's still essential to match the rhythms manually, and you should really know how to do it for all kinds of reasons, as was said at the beginning of the piece.
To move to the next level, you'll need to master the use of the tone control slider as a means of equalizing the tempo or rhythm. To demonstrate the effect of a rhythm mismatch, adjust the tone control of the second deck around the mark of more than 2 to 5%. Once you've mixed a song with the lock turned on, if it sounds a bit exaggerated (usually because it's just a few BPM away from the recording speed), use the tone attenuator to slowly return it to the BPM at which it was recorded. Cheap decks (especially belt-drive ones) usually don't have a very stable mechanism, which can cause fluctuations in pitch and tempo during a track.
As always, this is something you can only learn by doing. Nobody can tell you how much you should change the tone, you just have to develop an instinct to do so that connects you to your decks and the music they are playing. Once you've gotten used to starting your record with exactly low signals, it's time to start working on your tone control technique. Remember to figure out what pitch setting to set in your cube deck to match the rhythm of the live deck.
If these things happen earlier when recording with signals (that is, a particular rhythm or sound appears first in recording with signals and then, a little later, in live recording), this means that recording with signals moves too fast and you need to slow it down a bit; reduce the tone control setting a little and try to synchronize them again. If you've exceeded your pitch, you can slow down the disc a lot by radically reducing the pitch (just so that the two tracks sync up quickly) and then raise the pitch as fast as you can to your target area. The amount by which you need to change the pitch will depend on the severity of the loss of sync; if the two tracks get out of sync very quickly, you'll have to apply a large pitch change to compensate. Some discs can also cause tempo changes, either as part of the music itself, due to a warped disc (which is usually easy to detect, since the needle moves up and down as if you were on a roller coaster) or due to faulty vinyl pressing.
Of course, you'll have to set up the pitch yourself before you start, so you'll obviously have an idea of how much you've changed the pitch and in what direction, but if you don't see exactly where the slider is, it makes it a little more difficult. This is actually cheating, since you usually won't know what tone control configuration to put your signal deck in, but it's okay to start with a simple example to get an idea of what's going on. The tone effect is often overlooked because it's hard to know what to do to make it sound good in the mix. .