I need help understanding flange extenders

I have read many different threads/posts about flange extenders and still am confused.

I am removing the vinyl floor in my bathroom and installing hardibacker plus tile over the existing plywood subfloor so I need to raise the flange about 5/8 to 3/4 in. or so.

My existing trap/flange is PVC.

One of the "U" slots where the "T" bolts go is broken
on the existing flange so that only one "wing" sits under the flange. The heads of one or two of the 4 screws that hold it to the plywood look corroded enough
that I can't make out the "X" anymore where the screwdriver should go. The screws themselves
might be fine.

The rest of the flange is in good shape and the subfloor
looks to be in very good condition.

Questions:

1. Am I supposed to remove the 4 screws to the flange and screw the flange extenders onto the flange all the way through to the subfloor? or do I somehow attempt to attach the extenders to the original flange? Or do I
attach the flange extenders all the way to the subfloor
through new holes in the old flange?

( I know to use silicone caulk or something between
the extender(s) and each other and the flange. )

2. Is the broken U slot going to be a problem? Do I need one of those repair plates? Do I need to be able to get the existing screws out to make it work?

( From other posts, it appears that if the extenders are anchored to the floor, it is permissible to use the "U" slots on the extenders )

3. Do I install flange extenders before or after I tile?

4. How closely around the flange and extenders do
I cut the hardibacker and tile (gap or no gap)?

5. Do I grout or caulk or nothing in the gap between the flange extender and tile and hardibacker?

I know this is a long list. Any help with any of the pieces would be greatly appreciated.

Do you have access to the floor underneath the flange? If so, the best thing is to just cut it out, raise it and install it ON TOP of the finished floor. It should be supported by the finished floor and screwed down through it. Now, it can be a pain to drill through the tile, so many people make slots when setting the tile so they don't have to drill later.

You could stack some flange extenders on and a repair ring, but if you're goin gto all the trouble of making a nice new floor, why risk things by having all of those pieces leaking or coming loose?

I agree that extending the pipe and putting a new flange on the new finished floor is the ideal solution.

Unfortunately, I do not have access from underneath.

Looking down from above, it looks like there is about
2 inches of PVC between the bottom of the flange
assembly and an elbow that takes the waste a different direction.

I am not even sure that 2 inches is enough to work with to cut off the flange and to put on some kind of PVC fitting to allow for extending the pipe.

How much vertical length between the flange and the next fitting (elbow) would a plumber need to be able to cut off the existing flange from above and extend the pipe? (I don't want to spend $75 to have a plumber
come out and tell me that it can't be done easily and that
he/she'd recommend flange extenders...).

IF you flange wasn't already broken and you need to raise it because of the new floor going down, extenders would make some sense (and they still do). You need to be able to anchor the repair ring to the floor, or there'll be nothing to hold the toilet to the floor. If the holes in the repair ring don't line up, drill through the old flange to attach it to the floor. If there is nothing to anchor it to (like if the subflooring is not actually under the flange), then you'll have to do something to fix that.

2" isn't enough to put in a coupling, but you probably could peel off the old flange, and get a long neck version that you should be able to reglue on there.

I'd wait for one of the pros to give this a sanity check, or do a search on that...since it has been discussed before.

I assume the flange is glued to the outside of the pipe? If it is a 4" pipe, you can get a new flange to glue to the inside. If it is 3", that isn't a good idea.

Are closet flanges solvent welded to the PVC riser?

I am reading the word "glue" often in reference to
closet flanges. Is this just another term for a solvent
weld or something different?

I have been assuming that the closet flange is
solvent welded. If it is just some kind of industrial
glue, the option of
"prying the old one off" and replacing it with
a long neck version is starting to look promising.

It is solvent welded, but the pros have suggested that if you saw through the fitting in a few places, you can pry the fitting off with something that will fit in that slot (i.e. probably a screwdriver). It won't leave a nice clean surface, but usually you can remake a new connection with a new fitting over the remaining pipe. Use liberal amounts of the "glue" when attaching the new flange.