I'm making the comments below as I read the thread, so apologies if I answer something that gets answered later on.
I am just going to refer to "willpower" as the stat that improves healing output, even though technically it is "willpower and healing power" just to save space when typing. For the purposes of healing they both work the same.
Any calculations that try to compare willpower to crit in the general sense are mostly worthless. Healing abilities are made up of a base heal (typically only affected by the ability rank, which itself is based on career rank and mastery points) and a bonus healing from willpower. The value of the base heal and the amount of bonus healing actually gained from willpower varies from ability to ability. Assuming you are just trying to work out the highest average healing output per second (IMO not a great benchmark...) then any comparisons must be done on a per-ability basis that correctly account for base healing and contribution from willpower.
The base healing is important as the critical heal multiplier increases the total heal value, not just the healing contributed from willpower, so for some abilities that have high base healing and almost no contribution from willpower, crit becomes the only thing that can boost the healing output.
.
My calc is that 5 willpower = 1 bonus heal, and that scales in a linear fashion up to the softcap. Crit, is worth ~ 9 willpower. That being calculated from the average heal crit of 1.4 (1.3 + random range of 20% ontop). So if you had gear that has 0 willpower and 1% crit or another piece that has 12 willpower, technically speaking the 12 willpower would outheal the 1% crit.
Firstly, 5 willpower with a 1.4 multiplier becomes 7 willpower, not 9 willpower. Anyway...
5 willpower = 1 "healing per second" (HPS), so 800 willpower is 160 (HPS). The HPS stat is meaningless on its own, but each ability has an HPS multiplier which determines how much bonus healing it gets from the HPS stat, much like how offensive abilities get damage bonus from the melee/ranged/magic DPS bonus (which is based on strength/ballistic skill/intelligence) with a per-ability multiplier. When calculating the non-crit healing value, the formula is:
(Ability base heal) + (HPS * ability HPS multiplier) = final heal.
Each ability has its own HPS multiplier and it works the same as the primary stat multipliers for damage abilities. Typically for single-target direct healing the multiplier is equal to either the GCD or the cast time (whatever is higher). This is reduced for AoE healing and HoTs, and AoE HoTs typically have the lowest HPS multiplier of all.
The crit version is:
((Ability base heal) + (HPS * ability HPS multiplier)) * (randomised crit multiplier) = final crit heal
The crit multiplier affects the base healing as well as the bonus healing from HPS. Whether it is better to pump stats into crit or willpower very much depends on the ability HPS multipler (which can actually be 0, meaning willpower does nothing) and the ratio between the base heal and the amount of actual bonus healing from HPS * multiplier.
.
lots of posts about group heal crit chances
The conversation about this is terrible. Assuming a full group of 6 people and the heal crit chance is 35% from the healer (I am ignoring the hidden base chance to be heal crit which I believe is 5%?):
Each time you use the group heal, each person has a 35% chance of being critically healed. This is one crit check PER PERSON, not one per group, and the result only affects that person. E.g. if I crit heal on Alice, then that does not automatically mean I crit heal Bob. Bob has his own chance to be crit healed. It is entirely possible for a single group heal to crit heal 0/6 people, 1/6 people, 2/6 people, 3/6 people, 4/6 people, 5/6 people or 6/6 people depending on how many of those random crit chance rolls end up succeeding.
Sometimes the healer will have procs which trigger on direct critical heals - including group heals - that affect the person who received the group heal (such as the Zealot/RP's +25% incoming healing tactic). For these procs/effects, the group member that was actually crit healed is the only one who gets the buff. Example: if I crit heal Alice but not Bob, then Alice gets the extra effect from the tactic but Bob does not. Having more people in the group does not increase the chance of other people getting the bonus as the crit heal chance is a per-person check, and the tactic only affects the person who was crit healed.
Sometimes the healer will have procs which trigger on direct critical heals - including group heals - that affects themselves (such as Restorative Burst, which gives the caster a buff that grants them AP per second for a few seconds). The chance of this triggering DOES scale up with the number of people in the group, as it will trigger provided at least 1 person in the group was critically healed. The maths for this is a bit unintuitive, as you actually work out the chance of not getting a single crit and then subtract that from 100% to get the chance of at least 1 crit. Assuming 35% heal crit chance, the chance of not getting a crit is 65%, or 0.65 in decimal notation. The chances of NOT getting a single crit are:
2 people in group (including self): 0.65^2 = 0.4225, or 42.25%. This can also be calculated as 0.65 * 0.65.
3 people in group (including self): 0.65^3 = 0.274625, or 27.46%. This can also be calculated as 0.65 * 0.65 * 0.65.
4 people in group (including self): 0.65^4 = 0.17850625, or 17.85%
5 people in group (including self): 0.65^5 = 0.1160290625, or 11.60%
6 people in group (including self): 0.65^6 = 0.075418890625, or 7.54%
So then, the chances of getting at least 1 crit to trigger something like Restorative Burst on the healer is:
2 people: 100 - 42.25 = 57.75%
3 people: 100 - 27.46 = 72.54%
4 people: 100 - 17.85 = 82.15%
5 people: 100 - 11.60 = 88.40%
6 people: 100 - 7.54 = 92.46%
It will never hit 100% unless your crit chance is 100%, but it can get very close.