You do not have to put in a termination date, you just do not run payroll - but for proof that you laid them off prior to April 1 (so that they would be ineligible for any of the special paid leave programs)
Now the requirements for forgiveness of the PPP loan is that you maintain for 8 weeks immediately upon disbursement a level of employment that satisfies the letter of the rules. It is not 100% and you have two different look-back periods to choose from (possibly at your own discretion) And for total forgiveness 75% of your loan must be spent on ANY employees for gross wage, vacay, sick(not national leave) bonus plus health insurance benefits.
You must use up the PPP loan within just 8 weeks. If it is not all forgiven it comes due after 8 weeks with a 6 months no payment required and then 18 monthly payments at 1% interest - still not a bad deal if you spend it all on mortgage interest and utilities and have to pay it all back.
As to rehires, above and beyond using 75% of the funds for payroll, forgiveness also hinges on rehiring to same level of FTE (full time equivalent) , achieving that level by June 30, 2020
ANY employees. Yes, if you hire brand new employees for the next 8 weeks you do need I9 and W4 but calling back to work temporarily laid off employees does not require new forms, although you may need to submit a new hire (rehire) form to your state.
If you also get approved for an EIDL loan you cannot use any of that money for
A. any of the covered wages during the 8 weeks of PPP
B. any national paid leave