"integral" is different for different folks. Our swing OT has been very important for this team for awhile now. Cannon was paid starting pay when he filled this role. With Solder and Cannon, the backup OT was very important. However, we had 3: Waddle, Fleming, and Croston; all played. So, no, Fleming wasn't integral to the team's success.
Obviously, when Cannon and Waddle were injured, Fleming was more important.
That being said, I would be very happy to see us pay fleming $6M-$10M AAV or so to come back. The cap can be worked out. The "real" contract would depend on the 2019 salary. The contract would have only $4M dead money after 2 years.
2018 cap - $3M
4 years
bonus $8M
2018 salary $1M
2019-2021 salaries TBD
possible bonus before 2020 season
some guaranteed 2019 money