Hi-
Looking for some info about booking a Lufthansa flight.
Some background- flying from EWR to Gdansk, Poland- a business trip which will be reimbursed.
Dates are not firm- but I believe them to be 8/23 evening to 8/31 afternoon.

I would like to earn the maximum miles, but not take advantage of the company that is paying for this- I’m looking for the shortest and least expense trip.
Found a Lufthansa flight that works great- but I am new to booking with Lufthansa.
(I was a OnePass earner, now a Mileage Plus account holder.)

Do I open a new Lufthansa mileage account? Or use my existing Mileage Plus account when I book?
Should I pay with my Sapphire card or my United MileagePlus Explorer card?

I have lots of MileagePlus miles- is it worth it to upgrade?
Can Miles and More be transferred to MileagePlus—and vise versa?

Thank you in advance