A lace pattern for sleeves on a cardigan, I am working with has a 6 row repeat - with each RS row starting and ending with a varying number of edge stitches -
i.e.
K2 *K1, yo, ssk, K1, K2tog, yo* K3
P
K4 *yo, K3* K1
p
K2, K2tog, *yo, ssk, K1, K2tog, yo, S2KP* yo, ssk, K1, K2tog, yo, ssk, K2
P

Increases are needed every 12th row - which will occur in the first row of the lace pattern:
K2, M1, work pat to last 2 sts, M1, K2...

What happens to the edge stitches on the following rows - should they just increase by one after each increase row, Until there are enough stitches to work wth lace pattern? And then at that time, would you still include the edge stitches?

Of course you can just do stochinette untill a full repeat will fit. It can be done nicer, though.

thankfully, in this pattern the increase and the decrease are direct neighbours. so it will work out relatively easy.

When you do lace, you increase and decrease the same amount of stitches in every row (with few exceptions) - the count of stitches will be the same at the end of each row.

in your pattern, the decrease comes right before or after the increase.

This means:

you work your lace pattern and work the rows above one another. Now you increase in row 13 (right?)

this puts one stitch on each side of the lace pattern. I would just knit it in the RS rows and purl in WS rows accordingly.

then with the next increase you will have to count what part of your pattern will fall onto these stitches.

you then do your increase / decrease routine of just that part of the pattern.

yes, you knit2tog or ssk - that makes 2 original stitches. then you do an increase. therefore you have 2 stitches again.

Like this the lace will go as much as possible from edge to edge.

just check your stitch count regularly so you are not accidently missing something. The pattern shall not affect the total number of stitches on your needles (when the row is done).

Yeah, I'd just leave the new sts instockinette until you have enough for a pattern repeat. With incs every 12th row, that may not happen until the sleeves are full length anyway.