1) I noticed on BT's website it says "Your Stay Fast Guarantee 450Mb" which makes me think the speed will be inconsistent, but it being true FTTP, shouldnt you always get or very close to the speed you pay for, as there's none of the downsides of FTTC like crosstalk, line length, etc..
Its down to contention on the PON, see point 2 below.
2) Does anyone know how much bandwidth there is for each FTTP splitter to share between peoples houses, how many people one splitter serves, and how many people would it take to saturate all the available bandwidth shared to a splitter for it to start effecting everyone on it?
Its 2.4Gbit per PON which can have up to 30 properties sharing it. (technically 32 but Openreach keep two spare)
I assume if a lot of customers sign up for 900Mbit on the same PON, they would have to consider upgrading to XGPON which has more bandwidth and putting the top users on that, if they start to drop below 450Mbit. But as not everyone will max out their connection at the same time, they're betting on that not happening very often or any time soon.
Its not like any other residential network can sustain every user maxing out their connection at the same time either, FTTC, Cable, FTTP, are all contended mediums - as in the theoretical maximum speed of the customers connected combined is many times more than it actually can handle. But the odds of everyone maxing out at the same time is slim to none. Even at the ISP their peers to the Internet wont be able to sustain every single customer maxing out at the same time, and this is why different budget ISPs have different performance sometimes, a more expensive ISP will be more careful to have overhead compared to their "average" measured usage of their users.
The only way to have guaranteed capacity is to pay thousands for a leased line, where the ISP will make sure you get priority.
3) What is the reason BT advertise 900Mbs instead of 1Gig? I understand they can't offer symmetrical speeds like smaller FTTP companies do because they have to serve a lot more people, but it seems like a missed opportunity to sell "1Gig" and match Virgin.
Almost certainly OFCOM rules, they have to advertise what the expected throughput would be for the end user and that generally will max out at 940Mbit (as that's about the best you get from a Gigabit ethernet port), potentially less due to overheads, so 900Mbit is a safe bet.