You claimed tickets costing around £100. That is wild speculation. There is no reason to believe London-Birmingham tickets on HS2 will be anything like so expensive - other, obviously, than for the anytime tickets which cost a lot more than £100 even today on the WCML.
Guaranteed seats... well, we don't yet know what the policy will be on HS2. But in general, as far as I'm aware,
today no-one gets a guaranteed seat on any train (other than the sleepers), although people who book in advance can get seat reservations on most long-distance trains. So I'm not sure why you'd think it's a big deal if HS2's policy on seats is the same as almost all of today's existing trains? As it happens, given the high speed nature of HS2 and the high capacity of the trains that will run on it, I wouldn't personally be too surprised if a decision gets made to only allow travel on the high speed sections with a reservation, which would effectively guarantee seats (especially since by the time HS2 is built, reservation technology will probably be way beyond today's situation) - but of course that is just me speculating. In reality it's impossible to know either way.
On the up side, at least you quoted the correct current official figure for the cost of HS2
Note though that that's the projected cost of the entire line - phase 1 and 2 - not merely the London-Birmingham section that we currently appear to be discussing.