1) Spinner Rules are in context of the arena they are run in. Grade 2 and above has no stated limit, other than what the EO sets. Grade 3 is 500rpm, or what the EO sets - best advice is to read the rules carefully and in detail, and learn how they apply to different EOs and events.

2) Its always best to double check your figures from a unrelated source if unsure on the calculation. However that sites confused me aswell as it appears to be american, and thus imperial - yet gives the answer in metric. This is why you shouldnt use imperial for anything scientific :P

3) No, tip speed may be limited by EOs (EO has final discretion and may vary between). However youll likely still get over 200mph before they start getting cautious.

4) There is no difference between blade type, if it rotates its a spinner. Some robots have bars, beaters, ect - they arnt discs but come under the same regulations. Best advice is to not try finding loopholes in the regulations as the EOs will likely see through that and ban it anyway. If your defence is that its in the wording of the rules, tough - they decide if you enter their arena or not.

5) Build it at featherweight scale first. That way you can learn the technology and structural requirements needed whilst also not breaking the bank