I have three issues with this second-hand Birdy (production year unknow, possibly circa 2015):

1. When starting from a full stop, there's a bit of lateral play in the stem that goes away once I'm airborne: How to investigate, and possiblly tighten something?

2. Its eight-speed SG-C6010 Nexus is very draggy — the wheel stops spinning very soon after I stop pedalling —, although it's supposed to be premium; The brand new (non-premium) SG-C6001 I installed on another bike recently is much better. Could it be that the cones are too tight, or the hub needs lubrication?

3. Since I'll use it as a touring bike, I'd like to increase its gear range with a double chainring:
a. Is a new bottom bracket required for the chain line?
b. Should I replace its Alfine chain tensioner with a small derailleur?
c. For the front, what clamp + derailleur + shifter would your recommend?

Thank you.