There are two types of Godsend abilities, the ones which will usually last for 3+ turns (4 if synthing its prefered type of synth) and the ones which will usually only last for 1 turn.
On the first ones although they last for longer than 1 turn they will not "stack" with each other and as such you will only be able to have one active at any one time and as soon as you activate one the previous one will fade. You will still be able to stack one of the first group with one or more of te second.
The second group of Godsend abilities, although will usually last for only one turn, these are able to stack with each other numerous times and even with one of the previous group. also worth mentioning is that these abilities will let you know whether they have succeeded or failed to activate, this leads me to believe that the first ones can also fail and you as a player are not aware of it.
This is mainly a look at the first (r10) godsend abilities crafters get and how helpfull these can be, as such they will (in my opinion) be all of the first group, lasting for 3+ turns. In my opinion there are three types of craft attempts, grind, quality on mats and quality on finished items which these abilities will affect.
Grind is usually caracterized by the use of Standard and Wait commands and usually we just want to make sure the overall synth is successful in the shortest ammount of time possible. This will require you sometimes, while doing synths 5+ ranks above you to save as much durability as possible to make sure you get to 100% progress.
Quality on Mats is when you try to get as high Quality as possible on items which you do not have the oportunity to use Touch Up (leves will most likely be on grind mode unless you are looking fo obtain mats from 100+ quality rate). Once again you want to save on durability, specially because when using Bold you will need to trade it off for quality.
Quality on Finished Items is when you try to HQ finished items which enable you to do Touch Ups. This will be an approach which will mix both Grind and QoM, personally I go for standard as much as possible unless I get a red orb or when I see I'm gonna be finished under 30-50 quality and then I will invest in some Bold commands.
Tender Touch
At a only 3 points cost and easilly obtainable at rank 10 Carpenter this is my Godsend ability of choice. What it appears to do is that regardless of whether your synthesis attempt fails or successeeds it will reduce the durability lost up to 3 points (personally seem it happen). This can easilly be seen when you fail a synth, sound and text tip match a fail, but you see a higher progress than durability loss on standard attempt.
This will reduce the durability loss and increase the possibility of perfect synth (no durability loss) which will then enable you to get further progress for less durability. This should also affect Bold (although I cannot be 100% sure) and as such be usefull to get more quality for less durability and help get HQ items.
Preserve
At he usual 4 points and easilly obtainanle at rank 10 Alchemy this would be my second choice of Godsend abilities. Basically it will keep the colour of the orb white whether stabilised or not and this offers you a better chance at successful synths.
This will offer you to get more successful synths and a better chance at getting the higher ratio of progress Vs. durability on Standard and enable you a better chance at an overall success. I've also experienced the use of Bold on "preserved" orbs with returns of up to 25 quality which will of couse be handy.
Fulfillment
At the usual 4 points and easilly obtainable at rank 10 Leatherworker this is my third choice of Godsend abilities. This will give you a better sucess chance at Standard, Rapid and Bold synth comands.
By being more successful you have a better ratio of progress Vs. Durability not only on white orbs but on all other as well (I would still not use it on destabilized/blinking orbs) regardless of the command you issued.
Maker's Muse
At the usual 4 points and easilly obtainable at rank 10 Blacksmith this is my forth and last choice of the must have Godsend abilities for all crafters. As Fulfillment but restricted to Standard command, this will give you a greater chance at successful synth attempts.
Once again, by being more successful you have a better ratio of progress Vs. Durability
not only on white orbs but on all other as well (I would still not use it on destabilized/blinking orbs) but this time on Standard attempts. This can be usefull as well when doing QoFI to safeguard as much durability as possible for Touch Up.
Masterpiece.
Some crafter will disagree but personally I think that at the usual 4 points and easilly obtainable at rank 10 Weave this is one of the best Godsend abilities when you're looking for all those elusive HQs. As with Maker's Muse, Masterpiece will give you a better chance at successful synth but this time on Bold command only.
On QoFI I will only use this (and Fulfillment) when the orb shows red and I'll issue the
Bold comand to try and get the quality bonus out of it but on QoM synths I tend to use it on every orb that is not blinking while I have durability to spare.
Everyone has their own preferences and there will be crafters that will prefer
Harmonize or other rank 10 Godsend abilities to some of my choices. There will also be people that from having high rank crafts which I don't also have experience with abilities I have no such experience.
I would like to promote a discussion about the various abilities and their usefullness in various situataions.