You can use (almost) all of your powers when disarmed and without implements.
ImplementDDI and WeaponDDI powers don't prevent you from using them if you lack the appropriate tool: they simply allow you to get a bonus when you have the appropriate tool.
When using an Implement (keyword) power you make an unmodified ability check (usually Wisdom-based for Implement Cleric powers) against the target defense (as per definition of attack roll).
When you hold a magical implement (e.g.: a holy symbol), you get to add the implement's enhancement bonus to both attack rolls and damage rolls with Implement powers (and a damage boost in case of critical hit).
Also note that you can take advantage from any implement you are proficient with (not only holy symbols). If your Cleric took the Arcane InitiateDDI feat, then he can use even magical staves, wands and orbs to improve his Cleric powers.
A non-magical implement does not provide any bonus (except for flavor or feat support) unless it is a superior implement. Superior implements cost more but provides a varied array of advantages (properties similar to those of weapons).
For example, the Astral SymbolDDI costs 18 gp but has the Distant (+2 range of ranged and area powers) and Energized (radiant) (+2/+3/+4 damage bonus with Radiant powers) properties. Its base version is non-magical, but it you can buy or create an enchanted version of it.
You gain proficiency with a single type of superior implement by taking the Superior Implement TrainingDDI feat.
Lack of proficiency with an implement you are using means that you gain no advantage from its superior or magical bonus and properties.
When using a Weapon (keyword) power while unarmed, you make an unmodified ability check (usually Strength-based for Weapon Cleric powers) against the target defense (usually AC).
However, unarmed attacks have no proficiency bonus, so you lack a thick +2 or +3 modifier.
When you wield a non-improvised weapon you get a proficiency bonus to attack rolls with Weapon powers.
When you wield a magical weapon, you also get the weapon's enchantment bonus to attack rolls and damage rolls with Weapon powers (and a damage boost in case of critical hit).
Lack of proficiency with a weapon you are wielding means that you don't apply the weapon's proficiency bonus and gain no advantage from its magical bonus and properties.
Some powers may require you to wield specific weapons or implements. In this case, they explicitly state this requirement in their Requirement line of the power block. This is usually found in Rogue powers, for example.
Unseen Servant Interacting With An Object
Per page 284 of the PHB, 5e, second paragraph of the spell description.
"Once on each of your tuns as a bonus action, you can mentally command
the servant to move up to 15 feet and interact with an object.
The servant can perform simple tasks that a human servant could do,
such as fetching things, cleaning, mending, folding clothes, lighting
fires, serving food, and pouring wine. Once you give the command,
the servant performs the task to the best of its ability until it
completes the task, then waits for your next command.
It is reasonable to rule "Yes, the Unseen servant can pour healing potion into the mouth of another party member" who is not actively engaged in melee combat(moving about, swinging weapons, blocking, dodging) since that is:
- No more complicated than pouring wine into a glass
- Within the complexity range of tasks described, and even simpler than mending clothes.
Note:
- The spell description looks to limit this to one bonus action per
potion delivered. Issuing "standing orders" and assuming that the
Unseen Servant will do it each round without further instruction
looks to be beyond the scope of this spell.
- With a Strength of 2, the Unseen Servant's carrying capacity is 30 pounds. (p 176 PHB) If a pint is a pound (good rule of thumb based on the weight of water) ... add X for what each potion container weighs to arrive at a case, a twelve pack, or a six pack of potions. Work with your DM.
- If using variant encumbrance, it can carry 10 pounds with no movement penalty, and up to 20 pounds with speed reduced by 10 to, and up to 30 pounds with speed -20. (p. 176 PHB). With a speed of 15, the max that it can carry will be a bit less than the 30 pound max. (Speed = 15 is based on 15 foot move in one turn from spell description). Again, work with your DM to determine how many bottles or flasks of healing potion is a workable load.
Your DM may rule (reasonably) that the Servant cannot pour the potion into the mouth of the party member if the member is actively engaged in melee. That "moving wine glass" (the PC's mouth) can be missed, spillage, etc.
Work with your DM. You should be able to arrive at "go" and "no go" situations where this service could be provided. I think it's a great idea personally, and would be open to that service at my table.
See also this discussion of the spell
Best Answer
Unseen Servants are not meant or suited to be used in combat. Their combat utility, if your DM even allows it at all, is extremely limited.
An Unseen Servant can lift 100 pounds max, which is consistent with a strength of 5, and likewise cannot attack or take up space. As such, carrying a heavy metal plate doesn't present much of a deterrent. At best, it would give you maybe cover against ranged attacks. But possibly a bolt fired into the plate would cause the Unseen Servant to drop it anyway. Using multiple Unseen Servants doesn't overcome these limitations and is hugely impractical besides: they have the equivalent strength score of a puppy, so you'd need a lot of them holding on to that plate to make it even slightly difficult, and you need to instruct them all one by one to do it.
Any creature wishing to do so could easily push the Unseen Servant aside due to its low strength; it cannot stop anyone from simply moving past the plate to strike (as it takes up no space and cannot make opportunity attacks).
"Dropping" alchemical substance on the ground likewise isn't going to do much; an alchemical flask that would explode when casually dropped from waist-height would be very dangerous thing and much more force than that cannot be exerted by an Unseen Servant. There's a good chance the bottle wouldn't even break if dropped. You are supposed to smash them against something, which is why they usually involve making an attack (which the Unseen Servant cannot do).
Plus, the target of the potion could simply grab it as the Unseen Servant approaches with it and any kind of contested check would automatically be lost because the Servant cannot make attacks and has a really really low Strength score. Odds are good that you'll be eating your own alchemical potion if you try this.
So in addition to not being designed to do any of these things, care has been taken to make sure that even if you try, the Unseen Servant will be very ill-suited for these tasks anyways.