Any question answered...

Definitely ignore it for any short (under 1km) segments, especially if it's an uphill bit after a downhill. For example, look at the overall leaderboard of this segment:-

https://www.strava.com/segments/4020943/leaderboard?filter=overall

I know I didn't average 550W for 1:20 there.

Also, it's only as reliable as their elevation model, which can be wildly out in various places (so something that is actually downhill is shown on Strava as uphill, which completely screws their estimation, and vice versa).

Not very clear. I suspect just different crank options. Anyway, one will absolutely not work with the other. As I said, GXP is 24mm and bb30 as the name suggests is 30mm. A bb30 crank will be physically too big to fit in a GXP bottom bracket and a GXP crank will rattle around in a bb30 one.
Bb30 is not threaded so won’t fit in your frame. You will need either a GXP bottom bracket if you have a GXP crankset or some sort of BSA threaded bottom bracket with 30mm spindle and tiny bearings to adapt to bb30, if that’s what your crank is. Rotor and a few people make things like this.
Again, you need to know what crank axle you have

Basically, you can't change how wide your crank is or how far out it sits.

Not true. Sram 30mm cranks which are bolted on both sides use the same spline shape as Cannondale Hollowgram and RaceFace Cinch.

Okay so you can buy a totally new axle and change it out. I'm sure this is not recommended by sram and also doesn't help if Seafish has GXP which hopefully they do considering BSA frame

Can I friction shift a clutched rear mech across 8 speeds?

Probably.

It would do wonders for your chainline

How do I go about teaching guitar?
Do I need a grade 8/proof of ability, crb checks? Or could I just stick an ad in the shops?
I would ideally like to teach complete beginners than how to play Metallica or summat.

And, who do I report a business to for recording audio on CCTV and subsequently sacking people?

Are there any subscription style services where you can get a set amount of photos printed and posted per month? Like uploaded to an app or something? I could only find Timeshel whch seems to be US only.

I have a very fresh chain (SRAM 1 1/8) on a 48/15. Only been used indoors. I won’t be using it indoors for the forseeable. I want to maybe use bike outdoors before then. I want to run 48/19. Chain too short. What do?

Do I:

1) Use spare links to build long enough chain
2) Remove chain, set aside with 15t cog for reinstallation at some time in future and install new chain and 19t sprocket
3) ???
4) PROFIT

I'm in California, it's the weekend- what should I do?

Bumseks?

2, ideally a Wippermann 1Z1 and just swallow the ugliness, because it's the best outdoor fixie biek chain.

My girlfriend uses the FreePrints app, which gives you 45 per month and you pay something like £4 for P&P

Go hang out at mash SF like it’s 2007?

Go diving in Monterey

Go to the House of Prime Rib (if you can get a table) and then spend the rest of the time sitting down and processing the red meat.

New day - I shall head to Santa Cruz after breakfast (06:14 here)

Probably.

Pretty much my conclusion too. Friction shifting off-road might be a bit dodgy, but I'm going to give it a go.

Chatbook?

Looks good, thanks.

I've been using Yahoo since forever, but recently a whole bunch of emails went missing, it's happened twice now. Months worth of emails. Support is pretty non-existent. Looking online suggests it's due to some problem with mobile apps - I have an android yahoo mail app on my phone, but most recently when some emails disappeared I was using a PC at the time and not using my phone. So I don't know if that's really a good explanation, but I've uninstalled the app in case.
So, question 1: Wondering if anyone has experienced this, or know what causes it?
Question 2: What is a good personal email to use? I've had a gmail for a few years and haven't used it much, but could do. I haven't really got on with the interface but I'm sure I could get used to it.
Question 3: Is there a way of backing up/ archiving my emails from Yahoo to another email?

I've been with google since 2005, always been impressed with their in mail search and other stuff that they used to depose most of the other email services with.
Obviously creepier these days, but we're on line, yeah?

How much space should one leave between forks and tyre i.e. how much does a wheel flex?
Fork is carbon disc
Rim is strongish 36h
I've got about 3-4mm either side of tyre. Too little? Thoughts?

I'm in Chicago next week with one free evening. What should I do?