Don't read into anything. The rules for the Strolenati Quest (and all forum guild quests) are exactly as written with the only requirement being that which is set plus putting the Guild Freetext on it. If there was a requirement to use the main citadel quest dropdown, we would explicitly tell you that and describe how to do it. (Which is a great idea come to think of it!) It is an honor system being reviewed by the Guild Leader of whatever Guild you join. For the Strolenati Guild Recruit Quest you need the sub that meets the criteria and then you tell me that you have it finished in the forum thread you started. I say hurrah and you are in.
The quest logging box has nothing to do with the forum quests. If you look at the list you will see that the Quests there all match an official main citadel quest. The ability to join old quests was requested enough that I made that available. You can only win prizes and glory for a quest when it is active, however anybody can still complete the quest for personal honor and the respect of the Citadel. Those quests that are fulfilled outside the original quest time period are identified in your profile so you can tell official vs. follow-up submissions. So there is no overlap between the Official Quest and the Guild Quests...
HOWEVER, you can double dip. You can submit the same sub to the main quest and the guild quests and it would count for both... separately. That would only be a timing or convenience thing. So if you submitted something to the dropdown main Citadel Quest, you can ALSO use that in the forum Guild Quest. They are tracked completely separate.
You could think of this in one of two ways:
1) You write a submission to fit a main Citadel Quest dropdown. You select it from the dropdown and it will show up in your Profile as a main Citadel Quest. Then, as an afterthought, you see if it fits a Forum Guild Quest submission. Sweet, you post in the Guild thread that you submitted a requirement and we all celebrate.
2) You write a submission to fit a Forum Guild Quest in order to join a Guild and get the graphic. You submit it and put in the required Guild Freetext as per the Guild's instructions and we celebrate your movement in the Guild. As an afterthought you check the main Citadel Quests and you see it matches one of those too. You edit it and pick the Citadel Quest from the drop down selection and now it shows up in your Profile.
I don't understand your exact question, but was hoping that the full description of the two different Quest Paths would help.