I'll take these questions point by point:
I assume that using a single technique at your level (e.g. a competent technique at competent) never leads to overstepping?
It does if you use them more than just once. Using magic consumes ether, the lack of which accumulates as you use magic abilities. I'd think of it as becoming more efficient as you grow in power from competent to expert and beyond. Abilities that are on your level of competence or expertise or mastery tend to consume much more than abilities that are less advanced. So using abilities that are on your level tend to consume much more ether than lesser abilities, and can lead to overstepping. As with all things, it depends on how you're using it, how careful your pc is being, etc.
How many spells at your level can you safely cast before you begin to overstep?
Again, very situational. How many may not be the right question? I think how many in how little time and with how little preparation would be a more appropriate phrasing, albeit rather ungainly way of putting it
.
Magic isn't something to be taken lightly in general, and those who do tend to accrue overstepping penalties far in excess of those who are more careful and economical with their magics. It's not an easy question to answer as every ability is different, with a different set of circumstances surrounding it.
Does using novice spells when you are more advanced ever lead to overstepping?
Yes, it can. However, you can use far more novice spells at higher levels of competence than you can while a novice. But it still costs ether and can still lead to overstepping.
I assume trying to use spells that are above your level always leads to overstepping? Do the spells work at all in such a case?
Yes, it always leads to overstepping and consequences. Sometimes the spells will work, but how well they work will also scale with the severity of the consequences. For instance, a Defier using Refinement (a master-level ability) at competent will likely suffer catastrophic consequences and require intervention or even a mod-bomb. Consequences that can have far-reaching effects on their career as a mage and how they use their power. Or they might just result in a dead mage. Not advised to try it, if you can help it, unless you're happy with death as a possibility.