Yes.. You need some way for the gas to escape... While Polyethylene is somewhat gas permeable... It's not permeable enough to degas within any reasonable amount of time.... (You are looking for weeks/months, not years...)
Those containers also hold some residual pressure.. which will be enough to keep that CO2 in suspension... and leaving it tasting bitter/sour.
It needs an airlock to let that gas back out... I wouldn't stick air locks in those flexible containers... or you might end up with a wine all over the floor as they deflate...
On Sulphite (Campden, K-meta..) -- the old rule is to add sulphite every 2nd racking... With wild yeast, it is far less sulphite tolerant than "Store" yeast... so the amount of sulphite that would preserve and stop infections in normal wine will stop wild yeast in their tracks... Of course, it may not matter much now that you are 3-4 months down the road... It also helps slow down oxidation.... Personally, I would add some.. if only to keep infections from happening.
Hope all goes well.