The doubt is over which proficiency bonus to use, if any, right? Is your intended implication that it should be the PB associated with the spell level, e.g. +2 for 1st-level, +3 for 2nd-level, or do you mean the PB associated with the artificer at the time they stored the spells, i.e. at least +4, or something else?
One might feel drawn toward the PB of the artificer, but then by that logic the DMG should have been focusing on the level of the caster required to create a scroll, not to cast the spell, seeing as surely a 17th-level wizard could have created a 1st-level spell scroll. The words in the DMG on that are "the level of the spell on the scroll determines the spell's saving throw DC and attack bonus".
Therefore, is it right that you are saying the effective PB should be +2 if a 1st-level spell is stored, or +3 if it's 2nd-level?