To sum up much of what's already been said:
This is your first trip, so don't stress out about everything having to work out to ONE plan. Have plans, but be willing to flex as needed.
1. Try to get an earlier ADR.
2. Go early to your 6:50 ADR (arrive by 6:30).
3. IF, check is paid by 8:15, you have a chance to get to MK in time. Best spot is by hub, but will be very, very crowded.
4. IF NOT done by 8:15, go to the Poly beach. Less hectic. Might also see Electric Water Pageant.
My thoughts:
Depending on how quickly your group moves, how many in your party, crowd levels, bag check, etc. you may not make it to a good spot for viewing even if you are done eating by 8:00, which seems unlikely. So, you may not want to even entertain the idea of getting to MK for Wishes...too much anxiety about it.
On the other hand, I would not go to WDW and NOT see Wishes from MK. We were there last week (Presidents Day Weekend). WE found a good spot off the hub, near Tinkerbelle's flight path, around 8pm (for 9pm show). By 8:30 the hub was not walkable without stepping on something or someone.
WE find the projection show on the castle (Once Upon a Time?) to be as spectacular as Wishes. We stayed in the same spot for that show (9:45).
After Wishes, the mass exodus will make moving out of MK nearly impossible. The monorail and boat launches will be backed up for 30 minutes.
Stay put in MK afterwards. Plan to move INTO the park, instead of OUT. Take some time to shop, sit, snack etc. Let at least 30 minutes pass and you will have a much less stressful night.