I've ridden a fair bit of A17 in the last year at different times of day and both weekends and midweek.
I don't find it to be too bad, but not fun. It is a "High Risk" stretch of road AFAIK.
I don't know where the two controls that you're trying to link up are. The lanes (NCN 1) from Kirton to the A17 are great. Very narrow, but almost empty and well surfaced. Just using the A17 to cross the Welland might be your best choice?
Then take the lanes north of the A17. Again, very quiet and well surfaced and you might be surprised how nice those roads are for cycling. I was hoping to use these roads a lot but the wind always spoiled that. It would probably need an info control.
I think you may need to account for potential traffic jams too, though I think that the A17 isn't especially bad.
As a possible alternative, I suggest going West of Spalding via Pinchbeck on the B1180 or the parallel lane north of that (very pleasant), then A151 south, 2nd R on L hand bend into the lanes
R then imm L at Traffic Lights
R@T
L@T A1175 to cross the Welland
R@rab to Cowbit, then you have either the B1352 then B1165 or a myriad of lanes to head further East
The A17 is a major truckers route. Allthough I don't think it's especially bad, I don't think it the best road to use for a high profile event with tired foreign riders.
Another option may be having a control in Spalding, though I suspect that will now involve a huge amount of work.