Bathroom Floor Replacement Questions

This might be kind of long, but I'd appreciate you reading it through. Thanks.
I am going to bite the bullet and again replace a bathroom floor in May.
I have replaced the top surface once, about 18 - 20 years ago when I didn't know what I was doing. Because I didn't then know what I was doing, I need to do it again. I've gained a lot of knowledge since, but am always on the look out for more.
Smallish bathroom, about 5' x 8".
Previously had really poor condition sheet vinyl over a particle board underlayment.
Previously, I pulled the toilet and stored it in the back yard ( makes a great planter!)
Stripped all the sheet vinyl from the floor up to the cabinate base, and the shower pan. Removed any lingering vinyl adhesive with solvent, stripping down to the particle board underlayment.
When I first replaced it, on the advice of the folks at Color Tile, I did small tile (12" x 12" sheets) of 1" x 1" tiles. Thin set with a good notch ( 1/2 inch). Right over he particle board.
Surprise. After a lot of years of wear, many of the tiles are coming up. Especially where there is some drip / moisture from the shower door when the door is open.
Long to short, I have to pull all the tiles and re do.
I know I have a shower door problem re leaks. That I can and will deal with before I do the floor. So, please ignore any shower door leak issues.
I also have a wierd bath room door access issue. The access from the master bedroom to the bath is a pocket door. The pocket door didn't work when I bought the house 30 yaers ago and still doesn't. The bedroom floor wall to wall carpet. The bath has the afore mentioned tile. The pocket door is both off its tarcks and won't clear either the WTW carpet, or the tile.
I am reluctantly willing to pull the wallboard off the bedroom side of the pocket door to fix. I am not willing to pull the wallboard off the bath side of the wall because that forms part of the tiled ( by the builder) shower surround, and I'd really not want to have to replace the shower walls. The tile on the walls still seem in good shape, and no leaks, etc down into the family room below. My theory is if it ain't broke, don't fix it.
I did have one leak onto the tile floor, from the toulet, after I did the tile. This was about a year ago next June. Inside the toilet the friggin' float arm broke off . The inlet valve thereafter opened, and the inlet toilet valve kept allowing water into the tank. The tank totally overflowed, with awter coming out of the lid. Of courese I was at work, and when I got home after "X" hours, here was water everywhere. I sut off the safety valve, got towels and the hop vac, and pulled up a lot of water Wallboard on the ceiling of the family oom, under the bathroom, took a hit. Oddly, the floor in the bath did not appear to suffer any additional problems, no tile de laminating, etc. No where near any datile damage near the toilet from the one time flood as compared to the damage by the leaky shower door.
So, my questions are:
1. Whats the conventional wisdom regarding thicknesses of underlayment / underfloor and cement board in a bathroom before a tile application?
Is it 3/4 ply (outdoor ply, marine ply, regulay ply, whatever?) plus 1/ 4" cement board.
Is it 1/2 inch and 1/2 inch?
Is it /8 and 3/8?
What has worked for you?
My gut thought is that I need at least 1/2" ply plus whatever.
I also think I have to do an autopsy of the current floor to find out how much room I have inder it, i.e. I need to know the thickness of the present particle board underlayment.
2. The Orange Borg sells lots of 1/4" cement board. Lurking here, I read a lot of negatives about cement board for bathroom underlayment. I hear a lot of positives about hardyboard as an underlayment. Around here (Portland, Or.) here there is a lot of favorable comment about Hardieplank siding as a replacement for the LP or whoever crappy mold and nushrom growing exterior siding.. Same company? Parallel product? Personal experiences with hardy board vs. cement board?
3. Because this bathroom is so small, I like the look of the small tile (1" x 1"). Is there any longvity benefit to the larger (6" x 6" or 12" x 12") tile. I can cut the small stuff with a tile nibbler. I think I'd need a tile saw for the 6x6 or 12x12. Any tile installation requires tile cutting.
4. Underfloor heat. I have gas fired forced hot air. There is one vent in the bath room floor. It gives adequate haet when I showere about 06:30 am, as the set back thermostat kicks the heat up about 06:00. Still whil the bathroom air is warm the floor is cold.
I have seen a lot of Home Time / This Old House shows with electric (not water, electric) grids under the tile for heat.
Systems, brands, thoughts, etc.
Thanks for reading this maybe too long missive. I really would appreciate your comments and suggestions.
--
Jim McLaughlin

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20 years for vinyl tile is awesome! Congrats!
I would pull all the old floor and use cement board and ceramic tile.
But honestly isnt it time to look at the plumbing? How old is your home? Drains plastic or metal?
You might be better off doing a complete bathroom redo. Since you home is getting older:(
Any plans on selling someday?
If so do a job that will appeal to others!
Things like GFCI outlets, exhaust fan, additional heat, floor drain so NEXT toilet overflow just goes down the sewer:)
Some put in a shower that has no step over, that way shower drain drains entire bathroom in flood situation.
just some points to ponder
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--
Jim McLaughlin

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Dang it, I hate it when I am the first responder to my own posts!
Three other issues / questions?
5. I know I have to pull the old floor tile. I think I pull it separately from the particle board. Thoughts?
6. I know I have to pull out all the old partical board. I can pull nails easily. Any other thoughts on punning the particle board?
I figue I need puller bars anfd such, and not a circular saw or Saws All, as I want to get the particle board up without cutting into the underlying joists.
7 The shower pan is a massve, heavy soro of tile feling and not fiberglass thing which I think is a kind of ceramic / cement artificial stone.
I really don't want to replace it, or remove it and reinstall it,
How d butt he new pl underlayment and cement board / hardyboad up against t / under it?
I have this sinking 9bad pun) feeling that I have to wait until I do the floor autopsy befor I know haw t do that, but I'd appreciat any "heads ups" flks can provide.\\
Looking forward to everybodyss intelligent suggestions as I approach this.
Tahnks again in advance.
--
Jim McLaughlin

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THe pocket door should work fine if you can get it back on it's tracks. If it's still obstructed by the carpet and/or tile, you just cut a half inch off the bottom of it.
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