If it s not too wide i d line the back of the counter with crown molding or wall trim of some kind paint it with a high gloss paint so that it s washable.
Big gap between counter and wall.
An obvious gap between a countertop and backsplash is unattractive and it can allow water to seep through.
The fix is to remove those counters and actually fix the wall.
If the gap is too wide to seal with silicone narrow trim molding can solve the problem.
If the gap cover is shorter than the depth of your stove leave a gap between the wall and the cover.
Set your compass to fit the tip in the widest gap between the countertop and the wall.
You ll need a drill with a no.
If you can t do that then you are in over your head with this house and it will do noting but bleed you dry paying for people to do work for you.
Remove the excess countertop up to the pencil line with a belt sander and a coarse 60 grit sanding belt.
This will give you say a perfect 3 8 caulk line with an irregular wall to counter gap.
Close the gap between counter and wall if the gap between the countertop and backsplash is more than 3 8 inch it s generally too wide to fill and is best handled with trim but you can try to close the gap to give yourself the caulk option.
Tiles are started 1 8 off the counter leaving gap for caulk.
Measure the depth of your counter and cut the cover if necessary.
If there is say an inch to inch and one half overhang across the front of your slab maybe the slab could be slid back say one half inch closer to the wall closing the gap enough that when you add a backsplash you could fill in the difference with foam backer stuffed into the space between back of countertop and wall and then use flexible caulk to close the gap would depend on how thick your backsplash is as to whether this might be a viable fix.
Many of the covers you will find will be of one uniform size.
Your tile person will take the largest gap and run a tape line using that as the thickness or the caulk.
Measure from the edge of the counter to the back of the stove to determine the correct size for the gap cover.
It s supposed to be behind the counters and cabinets.