UC Deadline Date, Medical Conditions and Evidence

Hi, I'm new to the forum so I hope this is all ok - I tried searching the topics but got overwhelmed. Thanks to all the brilliant people helping e/o on here!

I'm on irESA in the Support Group for multiple physical and mental conditions, and have been notified of my migration deadline to apply for UC. I wasted a month in illness and depression, and another month trying to sort out ND assessment referrals with my GP (still trying!) - now I'm just under a month to my deadline date.

1st Question: Do I have to claim BEFORE the deadline date, or is it legit to claim ON the deadline date?

The whole B and W migration guide phrasing is "before" the date throughout, and I'd previously assumed I could claim on the date itself.

My deadline date is the Tuesday after Easter weekend. I don't understand if my final date to claim is the Thursday before the triple bank holiday, the bank holiday Monday, or the Tuesday which is my stated deadline date.

Any clarity would be appreciated!

Question 2: What kind of medical supporting evidence do I need to provide, and does this all have to be sent in before/on the deadline date?
Do I have to provide proof of all my medical conditions, upcoming appointments etc? I know I shouldn't have to do a WCA but not sure what evidence I need outside of that, and when I need it by.

Question 3: Are newer diagnoses of additional conditions a change of circumstance, even if my abilities remain the same?
I have extra diagnoses since I last had to reapply for irESA, it's taking decades to get to the bottom of everything that's wrong with me. If I list these additional ones, does this count as a change of circumstances and invalidate my transitional cover? Am I committing fraud if I don't mention them?

Clarification: I'm not on PIP because when I was on it I had to reapply and a traumatic tribunal experience + CPTSD made it impossible for me to reapply for many years. I should be getting it really, but from the B&W guide it looks like I lose my transitional cover if I apply for PIP after I claim UC (and I can't cope with applying before my deadline date). Is this correct?

Thanks for your help and sending love and solidarity to everyone out there!

David, thanks so much for this speedy and definitive reply!!

I am so relieved by your responses, and I'll be sure to update with my experience/outcomes.

Yes I am single with no dependents.

Please could I also ask, to verify ID I only currently have a passport, but I assume I will get a P60 soon too. Do you think I should wait to set up my account until I have my P60? I've gone through the UC form questions linked to on the B&W migration guide, but obviously don't want any surprises.

(I must have misunderstood the PIP situation - maybe I was thinking of the sever disability premium. It's so much info to juggle!)