I agree with your logic for the most part - and I am not 100% sold on the Okung signing, especially if he is going to command $8-10 mil/season like it appears. However, the way I see it is the 'benefit' of signing Okung would be that you don't have to then use that 1st round draft pick on an LT.
You have now freed up that pick to plug up another hole on the roster, which cannot be filled in FA.
If we don't sign Okung, I think you are absolutely locked into LT at pick #16. There really is no other option. Current Mock drafts show that 3 LT will prob be off the board by that time, so the Lions will get the 4th 'best' LT available. Sure it will still be a good player, but I am not sure that would be the best value for the pick, all things considered. If we lock up an LT, we can literally draft any position at 16 - more importantly, we can trade down a few spots and get more picks if the opportunity presents itself.