First a little background info about the house:
2-story, circa 1945, with basement
Public sewer
Cast iron main line (2 lines?)
2 of 3 toilets new within last 5 years
Mature trees in the neighborhood

Here was/Here is the issue:
Constantly stopped up 2nd level toilet; now tub/shower and sink slowly drain too.
(Other 2nd level toilet does not seem to be affected (2nd drain line?)
Now the 1st level laundry room toilet overflowed/seeped out through seal (while husband showered this morning), the drain in the laundry room gurgled, and kitchen sink 1st floor gurgled too.

And what I've tried:
Converted new low-flow toilets to full flush again
Snaked toilets (1st floor and 2nd floor) and the vent line from roof
Had Drain Doctor in and they snaked toilets too
I've used root dissolver (or whatever it is called) and various unclogging stuff
Used tub drain catch to keep hair etc. from clogging lines
Used bleach (about 1/2 gallon in each of the drains -- tub, toilet and sink -- for about 3 days) to unclog lines recently (recommendation from my dad as a good dissolver)

My diagnosis (am I right, or at least close?):
The clog seems to be below the first floor now.

Recommendations?
The house has been blessed, so anything else you can recommended?
Would bleach down the first level toilet, sinks, and laundry room floor drain, be wise now, or will it just push the clog further along the line?
Bleach in basement floor drain?
Any recs. Short of replacing the main line?
Any of the above really useless, so I don't do them again?

Is there a Saint of plumbers I should ask for intervention?
Thank you in advance for any help you can give me!

According to your description... sounds like it is time to lift the toilet on the first floor and snake the drain line out from there. You said you snaked toilets... but not the drain lines... right?

OR, may be able to find cleanout in basement just at the base of the stack going upstairs... try to flood the pipes to confirm clogged after the cleanout, then snake the drain from there.

When I snake from the first floor drain (toilet removed) I will usually run the water to a point that it almost backs up and out of the drain... now I snake the drain and when I hit the problem area I know because water starts to bob up/down. Then I usually push forward a bit... drain releases with a big "whoosh" because of all the water in drain... now pull snake back and see if something on it.

If nothing, then I snake to same spot as last time, continuing to work the drain/clog until I retrieve something or until I am satisfied I have cleared the blockage.

It does not sound like your MAIN DRAIN line is clogged as water would be backing up and out of the floor drain.

Bleach will not help to clear this blockage.

Anyway, that is what I would do at this point... hope that helps... Mark

First a little background info about the house:
2-story, circa 1945, with basement
public sewer
cast iron main line (2 lines?)
2 of 3 toilets new within last 5 years
Mature trees in the neighborhood

Here was/Here is the issue:
Constantly stopped up 2nd level toilet; now tub/shower and sink slowly drain too.
(Other 2nd level toilet does not seem to be affected (2nd drain line?)
Now the 1st level laundry room toilet overflowed/seeped out through seal (while husband showered this morning), the drain in the laundry room gurgled, and kitchen sink 1st floor gurgled too.

And what I've tried:
Converted new low-flow toilets to full flush again
Snaked toilets (1st floor and 2nd floor) and the vent line from roof
Had Drain Doctor in and they snaked toilets too
I've used root dissolver (or whatever it is called) and various unclogging stuff
Used tub drain catch to keep hair etc. from clogging lines
Used bleach (about 1/2 gallon in each of the drains -- tub, toilet and sink -- for about 3 days) to unclog lines recently (recommendation from my dad as a good dissolver)

My diagnosis (am I right, or at least close?):
The clog seems to be below the first floor now.

Recommendations?
The house has been blessed, so anything else you can recommended?
Would bleach down the first level toilet, sinks, and laundry room floor drain, be wise now, or will it just push the clog further along the line?
Bleach in basement floor drain?
Any recs. short of replacing the main line?
Any of the above really useless, so I don't do them again?

Is there a Saint of plumbers I should ask for intervention?
Thank you in advance for any help you can give me!
Mark, Thanks for your response. I hope you get to read this update and BIG THANK YOU! I did what you suggested and it seemed to alleviate the problem for almost an entire day...

The next day (day before lots of company arrived), my father-in-law and I tackled the clean out. Had to drill through the clean out cap/plug to get it out. Thought we augered it over 40 feet, past the house. Seemed good for about a week -- then the sewage came up through the floor drain on the first floor.

Dug around the back yard, found another clean out, and augered that... A big satisfying whoosh! SO AGAIN, THANK YOU for your help!

Glad if we helped some.

Thanks for the update!