Are you showing ammonia? If not you can try to combat nitrite using salt. the problem is the amount of salt one needs to add is a function of of the nitrite reading. You are really not wanting the salt perse, what you are after the the chloride part of sodium chloride (salt). Common table salt is about 2/3 chloride and 1/3 sodium.
Without going into detail, salt blocks the effects of nitrite in the fish, it does not change, neutralize or detoxify nitrite in the water. the problem is the needed level of chloride is 10 times that of nitrite. So you have to test the nitrite to know how much salt to add. In a tanks its easiest tow ork with mg/l for the salt since one must add weight to volume. If one know how many litres of water are in the tank then you need to add 1.5 mg of table salt per litre.
In your case you are in a bind. It appears as if you have the ammonia part under control since you do not report ammonia readings. You are building the nitrite bacs back up. Changing big water slows this process since the amount of nitrite being created will not drop- it comes from the amount of ammonia being produced. So you need enough nitrite oxidizers to handle that much nitrite. By changing big water to hold the nitrite level down you retard the process.
I can not tell you that leaving will not result in big build up and serious problems. I have no real idea what level of salt you might need to add before you go because you, and I, have no idea how high the nitrite level actually got. The amount of salt needed is not huge but it needs to be the proper concentration in the water since many fish dislike salt as do many plants. You don't want to "over dose" it when you are not there to monitor things.
I can not suggest what to do here since I do not have a firm grasp on what is going on in your tank.
And just an fyi- in a properly set up under gravel filter almost all the bacteria is in the gravel and not in any other filter one may also have on the tank. UGF and RUGF are still the single best bio-filtration out there. However, they do not work well in certain setups and they require proper maintenance. But where they can be used, they can not be beat for bio-filtration.