1. Depends on the engineer sent.
Openreach ones tend to install a new-style of filtered socket (which is a kind of interstitial plate between the existing master backplate and the existing half-sized frontplate) into an existing master, putting the right kind of master in if needed. However, they're starting to leave filtered sockets if they're already in place.
Sometime contractors turn up, who tend to leave existing setup as-is.
If you ask for the data-extension kit, then these tend to be installed better by Openreach engineers, while contractors seem to be keener to not do them. You should tick the box to ask for this anyway because, even if you don't end up with one, it seems to allow the engineer a little more freedom to alter or move the existing master instead.
(i) Ethernet cable, RJ45, not crossover, length is standard ethernet limit (up to 100 metres of solid core, or around 30 metres of flexible patch cable)
(ii) RJ12, similar to existing ADSL cables. Best kept short, but if needed, can be longer - but a quality one with twisted pairs should be used.
2. Are there other extensions? Where are they wired from? What quality is the cable from existing master to extension on 1st floor? Is a phone socket needed at the existing extension?
The two main solutions are:
i) Existing master turned into filter. BT use a data extension kit to replace the existing extension upstairs and the wiring to it (uses Cat 5-style cable instead of std phone cable). This would lose the phone socket upstairs, and can't connect any further voice extensions from it.
ii) The extension upstairs can be turned into the new master, with the filter placed there, giving both types of socket.
The existing cable can be used to carry the BT line up there, provided it is good enough quality (ie not cheap flat untwisted cable).
If it has enough pairs, it can also carry the extension wiring back down to the existing master, which becomes an extension. Any other extensions from here can be included in the wiring. If it doesn't have enough pairs, then a second cable needs to run back to the existing master.
Not every engineer seems to like this second style, particularly contractors.
3. Can't help with this.
4. In the past, predicted speeds have been very pessimistic. However, as take-up increases on a cabinet, those initial high speeds tend to drop back towards the estimates because crosstalk increases.
Contention ratio isn't a thing ever talked about any more. But there doesn't seem to be much issue with congestion of the backhaul from cabinets, or within the core network (both within BT Wholesale), or in the pipes to Plusnet (which have shown occasional problems for Plusnet, but have always been looked at)
5. I've always been a fan of the prioritisation aspect of PN's traffic management, which does indeed appear to be a sensible feature. If you're going for the unlimited package, then this is all you have to worry about.
I'm on the older variant of the "Extra" package, which still has allowances, but the same speeds limits as the newer "Unlimited" package (ie none). I haven't had a single problem with it - even in the early days when there were some limits.