In terms of galvanic corrosion with magnesium, Titanium is worse than steel or stainless steel. The closer the two metals in contact in this chart the less galvanic corrosion will happen. And titanium and magnesium is very apart from each other (about 1.5 volt difference between the too):
For galvanic corrosion to happen there has to be electrolyte solution between two dissimilar metals. Road salt mixed with rain, snow slush is one.
Because the bolt heads are pointing down in this case, the salted water will drain won't not stay. Also there is aluminum between the magnesium and the bolt's exposed head with these covers. That may be reducing the effect too.
The stainless steel bolts may have some plating on them that would reduce the contact and also corrosion as well, but it is not specified.
Overall it is better to use aluminum bolt, I think.