Can this be used for spot repairs? I have a pool with a ten year-old liner which is in great shape except certain areas are starting to pull away.

It depends on the type of pool. I have a beaded liner and it was not thick enough- you'll need liner blocks. If it's an overlay liner I don't know.

I just had my liner replaced 4/25/2022. It has a built in liner lock. Can this product be installed over / in addition to the already in place liner?

Yes, but the liner lock product in question is really not necessary for a new liner. However, if you live in an area where heavy rains occur from time to time, it would be a good idea to install a liner lock beading as a precaution. Extended heavy rain can cause the water table to rise high enough to float your liner and push it off of the track at one or more places around the pool. For liners that are several years old, I definitely recommend installing a liner lock beading.

My pool guy told me to install the liner lock in 1-2 ft sections, as it can expand and pop out of the coping. Has anyone had this issue?

It was easy with 2 people. One person putting and holding it in place while second person tap it into place with hammer and a small bite. It's been in a year now and has not popped out at all.

How wide is the front and how deep is the insert?

The profile is 'T' shaped . The 'cross' of the T - is 1/4 inch (quarter inch) . The vertical is 5/16 inch ( Five sixteenths). Note : I purchased a roll but discovered my Liner had the necessary 'tightener lock' built in to it.. Just as in the photo to the left - but without the locking 'T ' roll.