You won't have to at Bank, as you've already touched in at Angel, and there's just a staircase down from the Northern Line platforms to the DLR platforms.

(One note: there are lots of engineering works in Bank station at the moment, making it extremely crowded in the rush and shoulder hours. There's a lot of recorded messages telling you to avoid the station where possible, but you can ignore these, unless you're particularly claustrophobic...)

Once on the DLR platform, if you turn right and walk along, you'll see a small, silvery bunker on the platform. If you stand by the platform edge level with this bunker, you'll be level with the front doors. The DLR is driverless, so it's particularly good fun to sit up the front!

I would do. When I used to commute on the Northern Line, I either went at 7am or 9.30am. At Clapham Common (my old local station), the trains used to run through at 150% capacity...

There are some lovely walks from Angel down towards the City of London in the morning (the main roads are busier and less pleasant). There's also some canalside walks you can do there to some nice pubs (just watch out for fast-moving bikes in the rush hour, and be careful after dark). From there down to Bank, as a roundabout route, you could wander down through the Chapel Market, across to Myddleton Square, down to Exmouth Market, down the back streets to Clerkenwell Green, over to the Whitecross Street market, a detour through Bunhill Fields if you fancy, then down past the Guildhall to Bank. Would take about an hour.

Yes, that should be OK.