Are MK points spent on using Dominion techniques one-time use (for example spending 5 MK for just a temporary boost to one attack)? If this is the case, it seems awfully expensive since you can gain only so much MK per level.

No, MK are used like DP, to buy techiques and Ki powers (Capabilities). Ki are expended to use techniques, but those refresh (base once per hour per stat, based on ki accumulation rate of that stat).

You can also get MK by investing in martial arts. If you have dommus exxet there is also a rule that you can buy MK at 5dp = 5km (I believe it is under primary points)

Not under primary, but in the Physical section, with a maximum of 10% of total DP

DPs spent on MK count as DPs spent on Combat Primary Abilities and yes, there's a 10% limit to it (for example, a Level1 Warrior character with 600DP might invest up to 60DPs for 60MK, but would then have only 300DPs instead of 360 to invest in other Combat Primary Abilities).

Sorry for my incredibly late reply (I have been very busy lately), but thank you for all your answers! I'll be sure to have more questions as I go through the core book.