The problem is, I do not know how conductive the "conductive" glues are. Do they introduce a few mΩ resistance? Is it a few, a few 10s or a few 100s? Is it enough to cause a noticeable voltage drop and hence problems at higher currents?

I personally would solder. The "battery" contacts are not actually directly contacting the battery, there is a tiny circuit board between the battery and the terminals. But I would be extremely careful not to overheat the board while soldering, as even the transferred heat might be enough to permanently damage the battery (trust me, I know - and I have been soldering things for 30 years ). For the less experienced, a glue is a less risky solution.