Planned tentative meeting with potential client, I'm planning to book refundable First class Delta ticket (Class P). If meeting is cancelled, I plan to fly back home on already confirmed ticket with another airline.

1. Flight is at 7.30 pm and I plan to cancel it by 3pm depending on response from client - Is P class full refundable?
2. I'm currently TSA pre-check member, I never booked 2 flight tickets leaving from same airport on same day in matter of 3-4 hours- will TSA question about these 2 flight ticket booked?

1)Fare class doesn't matter. Read the fare rules.

2)TSA won't have a clue. The BP scanner isn't connected to the airline systems. It just reads the barcode and verifies it has the correct signature.

I agree they would have no clue about the other flight. But the scan will show pax name, flight and such. Everything that is on the BP barcode (or QR code) in a standard format.

I assume that they track these. Not because it really matters, just because that is governments think.

Yes, make sure to click through to read the fare rules (details or summary should be okay). As DL warns in its "Refund Policy" - "A fee may be required to refund or change some refundable fares." You want to see the text "Cancellations Permitted", usually under the "Penalties" section. If there's no fee listed right there then you're good to go. Also, you shouldn't technically need to cancel in advance on a fully refundable fare; if you do the fare rules will state that in the Penalties section as well.

Lastly, print and save the fare rules just in case!...

I've seen some domestic FC P fares that are fully refundable (with no fees charged) and other P fares that are nonrefundable, sometimes for the same city pair. OP should be very careful.

I second the advice to be sure to print the fare rules, both the summary and the detailed rules that you get by clicking again on the fare code. If there's more than one fare code for the ticket (unlikely since it will be one way, but DL can show broken fares without warning the customer), do this for every fare code you see. Remember that (unlike some other airlines) delta.dumb does not have a way to see the fare rules after tickets have been purchased.

I purchased a "Refundable" ticket on DL with a fare code IFFWTZ. It says refundable under the fare code, but in looking at the fare rules I find no mention of an amount charged to cancel. This is for a one-way JRO-MIA in Business and all flights are on KLM metal ($1,900). Is it really 100% refundable for any reason at any time or is there a way to find out the amount if not? Is the PENALTIES the relevant section?

Voluntary ChangesIN THE EVENT OF CHANGES TO TICKETED FLIGHTS BEFORE DEPARTURE OF JOURNEY - APPLIES WITHIN TKT VALIDITY CERTAIN DOMESTIC REISSUE PROVISIONS MAY BE OVERRIDDEN BY THOSE OF DL INTERNATIONAL FARES NO CHARGE FOR REISSUE OR HIGHEST FEE OF ALL CHANGED FARE COMPONENTS - CATEGORY 19 DISCOUNTS APPLY AND REPRICE USING FARES IN EFFECT TODAY PROVIDED ALL OF THE FOLLOWING CONDITIONS ARE MET- 1...
Flight ApplicationTHE FARE COMPONENT MUST NOT BE ON ONE OR MORE OF THE FOLLOWING ANY QR FLIGHT ANY LH FLIGHT ANY UA FLIGHT ANY EI FLIGHT ANY EK FLIGHT ANY EY FLIGHT ANY MN FLIGHT ANY W3 FLIGHT ANY SW FLIGHT
Advance Reservations and TicketingRESERVATIONS ARE REQUIRED FOR ALL SECTORS...
StopoversUNLIMITED FREE STOPOVERS PERMITTED
TransfersUNLIMITED TRANSFERS PERMITTED ON THE FARE COMPONENT ONLINE ON THE PRIMARY CARRIER...
CombinabilityEND-ON-END NOT PERMITTED...SurchargesIF INFANT 0-1 WITHOUT A SEAT...
Sales RestrictionsTICKETS MAY NOT BE SOLD IN VENEZUELA/CUBA/ZIMBABWE/ CAMEROON/CHAD/CONGO (BRAZZAVILLE)/GABON...

PenaltiesCANCELLATIONS CANCELLATIONS PERMITTED
.
..Higher Intermediate Point(HIPS)/Mileage ExceptionsORIGINATING EAST AFRICA - THE HIGHER INTERMEDIATE POINT RULE DOES NOT APPLY FOR STOPOVERSChildren DiscountsACCOMPANIED CHILD 2-11 - CHARGE 100 PERCENT OF THE FARE...Tour Conductor DiscountsNOTE - DISCOUNTS APPLYAgent DiscountsNOTE - DISCOUNTS APPLYMiscellaneous ProvisionsNOTE - PASSENGERS WHO HAVE A CONFIRMED RESERVATION S FOR TRANSATLANTIC TRAVEL VIA DL BUSINESS CLASS SERVICE WILL BE PERMITTED TO MAKE RESERVATIONS IN THE FIRST CLASS COMPARTMENT OF DL FLIGHTS WITHIN AREA 1 PROVIDED NO BUSINESS CLASS COMPARTMENT ON SUCH FLIGHT WITHIN AREA 1ApplicationBUSINESS CLASS FARES APPLICATION AREA THESE FARES APPLY BETWEEN THE UNITED STATES AND AREA 2/AREA 3...

Yes. If it says "Cancellations permitted" in the cancellations section, without any fee or penalty listed, then there is no fee or penalty to cancel.