With the final update Wounds of Eventide, it's possible to sequence break and skip the first four real bosses of Blasphemous (Ten Piedad, Tres Angustias, Our Lady of the Charred Visage, Esdras). This might still be changed/fixed since it breaks the game, especially since it's achievable on a new save, but it was fun to attempt and I thought I'd share my notes.

You can get from Echoes of Salt to Mother of Mothers through the difficult new area Mourning and Havoc.

You will need the following (on a new save):
- Blood Perpetuated in Sand relic, found in a locked room in the Brotherhood of the Silent Sorrow, opened with a key bought in Mercy Dreams for 400 Tears.
- Tirana of the Celestial Bastion prayer, found in front of the chapel in the Brotherhood of the Silent Sorrow. Used to bring down the ladder in Mourning and Havoc to get to the boat. Needs ~4.5 fervour bars. (You start with 3 bars)

You will have, at most, the following upgrades available to help you get through Mourning without going into any boss rooms:
- 2 Fervour upgrades (Grievance Ascends, Graveyard of the Peaks)
- 4 Flasks (Mountains of the Endless Dusk, Jondo, Graveyard of the Peaks, Mercy Dreams)
- 2 Quicksilver (Petrous, Jondo)
- 2 Mea Culpa upgrades (Albero, Brotherhood of the Silent Sorrow)
- 1 Health upgrade (Graveyard of the Peaks)
- 3 Knots (Jondo, Grievance Ascends, Graveyard of the Peaks)

A Medium Bead of Blue Wax might substitute for 1 fervour upgrade. Found first at Convent of Our Lady, then at the Brotherhood of the Silent Sorrow.

These rosary beads may help you survive:
- Ember of the Holy Cremation (medium defense, bought for 5000 Tears in Graveyard of the Peaks)
- Perpetua's Protection (electric resistance, must defeat Perpetua in the Mountains of Endless Dusk, you have 1 chance to get it in this route)
- Fourth Toe of Limestone (dodge cooldown reduction, Jondo)
- Medium Bead of Red Wax (hp boost, Mercy Dreams, then Convent of Our Lady)

Finally, you'll need patience and/or a familiarity with the mid-air slash jump, so you can get more quick hits in between dodging the electric projectiles of Mourning and Havoc's unavoidable and tanky new enemy. You will need to defeat 2 of them, and their medium speed projectiles will follow you for a while until they hit you or disappear.
The 2nd one is close to an anchor-throwing enemy which can be stunned into Execution mode with a sword Thrust, so that sword skill may be useful to purchase. After clearing the platform, you can jump off the edge here and do a few mid-air slash jumps at the enemy on the right, then jump back towards the ladder and jump-climb up to reach the platform again while avoiding its projectiles.
After that, there are a few tricky jumps/ledge grabs on the final stretch on the wooden beams that include the area's most rapidly cycling electricity pulses and the anchor-throwing enemy, which all depend heavily on timing. It's good to have some flasks left for this spot. The timing for the electricity pulses' hitbox is slightly different from what you'd conclude from its animation, you can try touching the ones near the save spot at different parts of the cycle to see what I mean.

Here's my well-explored map at the end of Mourning and Havoc, without going to any boss rooms:
It's definitely possible to get there with less preparation, but it'll take patience and some practice.

Using this route, you only need to defeat 6 bosses to beat the game (including the tutorial boss).

Some maybe unimportant caveats:
- Upgrades within Desecrated Cistern will be inaccessible until you get the Linen of Golden Thread relic.
- You can't tithe at the church unless you defeat one of the first three real bosses, so no teleport, Token of Appreciation, or free fervour.
- Can't get the Knot from the medicine quest.
- Can't get the new ending this way.

Enjoy! (while it lasts?)

Underline = necessary steps

From new save, go to Albero, get Mea Culpa #1
Go to Mercy Dreams, get Red Wax #1, buy Key, get Flask #1 in hidden section

Go through Olive Trees to Graveyard of the Peaks, get Knot #1, get Flask #2, fill Flasks, get Fervour upgrade #1
Go to Our Lady, get Red Wax #2(long detour), ride elevator to Graveyard, get Health upgrade, ride elevator to Albero

Go to Mountains through breakable wall in Albero, defeat Perpetua & get rosary bead
Go to Jondo, break right chain, get Quicksilver
Go through Echoes, open shortcut to Mountains, Quicksilver upgrade one time

Go to Jondo, get Flask #3, fill Flask, break left chain, get Knot #2, open shortcut back to save spot
Go to Brotherhood, get Mea Culpa #2, open Hand portal, activate Knots, get Blood in Sand relic, get Tirana prayer

Go to Mountains, get Flask #4, get Fourth Toe
Go to Jondo, break floor to Grievance, open Hand portal, get Knot(quick jump, grab & reload game), fill Flask, get Fervour upgrade #2

Hand portal to Brotherhood, buy sword Thrust skill, activate Knot
Hand portal to Albero
(optional hand in bones to Ossuary & herbs to Tirso for Tears, ride elevator to Graveyard, buy Ember for 5000)

Go through Mountains shortcut to Echoes, fill any remaining Flasks
Go to Mourning, drop ladder with Tirana, ride boat
Go through Mourning to Mother of Mothers!


Disclaimers:
- There will be faster routes for the exact number of upgrades that you personally may need for surviving through Mourning.
- I haven't tried this exact route
- There may be mistakes here
- In this sample route you will need to pay for the Key (400), filling Flasks (500+750+900+1250), a Quicksilver upgrade (1000), sword thrust skill (1500), and the Ember rosary bead (5000), totaling 11300(!) Tears.
- If you can make it through Mourning with fewer upgrades, you will save a lot of time.

Good luck if you try this, hope it's helpful!

This one's shorter & doesn't pass Grievance, but includes only the necessary steps:

Go to Mercy Dreams, buy Key
Go through Olive Trees to Graveyard of the Peaks, get Fervour upgrade #1
Go to Our Lady, get Blue Wax #1(starting one), ride elevator to Albero

Go to Mountains through breakable wall in Albero
Go through Jondo to Brotherhood, get Blue Wax #2, get Blood in Sand relic, get Tirana prayer

Go through Jondo & Echoes to Mourning, drop ladder with Tirana, ride boat
Go through Mourning to Mother of Mothers!