Be aware as well that once you pass your first exam, you have 18 months in which t complete the rest and then ( I think) 6 months to take your skills test after that.
No, you have 24 months in which to take the Skills Test and apply for the licence from completing the last exam. Perhaps more important is that you now have to pass all exams in 6 sittings including any resits. A sitting is defined as a 10 day period and any exam failed cannot be retaken in the same sitting.
Of those 100 hours how many have to be in the classroom?
That is up to the ATO to propose to the CAA, and for them to approve it. It will not be a fixed amount.
Can I even sit some of the exams before working towards the 45h mark for the PPL?
You can take the exams at any time bearing in mind the validity periods. Also at an ATO you will need a sign off to take the exams but not at a RF. If you hadn't noticed there are now Apple and Android Apps for all the exams. They are no harder than previous exams but cover far less relevant corners of the very broad and ill defined syllabus.