Considering the Number of Lines

If the commerce reforge also modifies other lines, wouldn't it be better to have fewer lines to make it more likely to get the commerce speed then use a credne to get other lines or would the credne overwrite the commerce speed line?

Your logic seems completely backwards here. Each line is rolled separately for commerce speed, thus, more lines means greater likelihood of getting commerce speed. Also, all lines are rolled (as in, you lose the previous buffs) when using any reforges, crednes included.

The odd thing is, I've used 5 of these on a 3 line outfit, and the strangest thing happens. I've gotten Commerce Speed Reforge every time, even though it shows up on another line. It seems like it is ensured, almost 100%, but considering I only used 5...hard to say.

I'm still trying to figure out how reforges work. The pages about them aren't terribly clear.

To reforge something, a Reforging Tool is required. If you are not satisfied with a reforge, you are required to use a Fine, Credne's, or Commerce Reforging Tool, depending on what is needed.

Items that come reforged with a blank Rank 3 (from Spirit Weapon Reforging Glitch wayyyy back when the Reforging Tool was first released), 2, or 1, a Fine, Credne's, or Commerce Reforging Tool is required, since it was already pre-reforged (preforged?).

Fine and Credne's Reforging Tools can "rank up" the reforge to a higher level through a cutscene. If the rank is already 1, it may "critical" reforge with the same cutscene, giving much higher and stronger reforges. Credne's Reforging tools can also add more lines (Up to 3), up to 2 more if there is 1 line, or 1 more if 2 lines exists. Credne's randomly add lines, while still functioning as a Fine Reforge. Just use Credne's reforging tools if you need more lines, or use fine tools if you already reached the 3 line limit.

And if you want a commerce speed reforge, stop using fines once you're r1 with three lines, then use a commerce tool. Higher rank of reforge, higher max effect level possible.

You mean Credne's. Fines do not add lines.

Credne's until three lines. Fines until r1 if the item is not already r1. THEN Commerce, is what I meant.