This is no problem at all. To address your questions:

>*"what effect (taste, strength, yeast effects) might I expect from adding the sugar at the start of fermentation?"*

Priming sugar (almost definitely glucose), being nearly tasteless and highly fermentable (90+%), will increase ABV% without adding either residual (unfermentable) sugar or any malt flavor, basically lightening the body of finished beer. The magnitude of this effect depends on how much sugar you added, as well as what the rest of the recipe looked like, but since the bag was intended as priming sugar, it won't have been too much sugar and the effect should be entirely minimal, if even noticeable at all. Yeast would only be affected if it was a huge amount of sugar you added (25+%, maybe), and this would be due to the dilution of essential nutrients derived from malt. You won't see a problem here.

>*"I have another bag of priming sugar, I still plan to add it prior to barrelling. This should be okay?"*

Yes, absolutely. The sugar you added before will get fermented fully before bottling, so it's necessary to add more to achieve carbonation. 

>*"Maybe I should take one reading and then another the day after to check that it's stable."*

Do this. To be safe, most people here recommend waiting until it really looks like fermentation's done, then taking gravity readings three consecutive days to ensure it's fully fermented. When this checks out you can proceed to packaging.

Your beer shouldn't suffer at all for this mishap, so don't worry too much about it. It's probably the least costly mistake you could have made.