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.
Big gap between counter and wall.
Measure from the edge of the counter to the back of the stove to determine the correct size for the gap cover.
Remove the excess countertop up to the pencil line with a belt sander and a coarse 60 grit sanding belt.
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.
If the gap is too wide to seal with silicone narrow trim molding can solve the problem.
If the countertop isn t level slide shims between the countertop and the support struts of the cabinet frame to level the countertop.
Tiles are started 1 8 off the counter leaving gap for caulk.
You ll need a drill with a no.
2 phillips bit a flat pry bar wood shims and a hammer.
The fix is to remove those counters and actually fix the wall.
Measure the depth of your counter and cut the cover if necessary.
The line will continue maintaining caulk gap regardless of the gap of the wall to counter.
Set your compass to fit the tip in the widest gap between the countertop and the wall.
Many of the covers you will find will be of one uniform size.
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.
If the gap cover is shorter than the depth of your stove leave a gap between the wall and the cover.
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.
This will give you say a perfect 3 8 caulk line with an irregular wall to counter gap.