Yes, you can cast prepared spells which require only Verbal and/or Somatic components when deprived of your focus. (As long as you can produce the Verbal and Somatic components, that is.)

The focus only functions to replace (subject to the limitations you mentioned) such Material components as are required. It, like a component pouch, is not implicated when a spell has no Material requirement.

Better way: simply just pretend to cast the spell

Before I get into answering your question as asked (and explaining why it won't work the way you want it to) let me propose a simpler, easier and less costly way of doing the same thing:

All you have to do is pretend to cast a spell while not actually casting anything. Take out components, do hand gestures, and start chanting but do not actually initiate the casting of any spell.

If you do that, and successfully trick a spellcaster into casting counterspell (or dispel) then you have made them waste a 3rd or higher level spell by expending no resources.

See this question for more on this.

No, incomplete spells do not have visual effects, so there is nothing to create an illusion of

Spells do not have any effects (visual or otherwise) until their casting is complete

You say:

So far I have seen no one describe the visual appearance of the casting of a spell any spell other than a DM before/after announcing the spell´s name and level.

And there is good reason for that.

To be perceptible, the casting of a spell must involve a verbal, somatic, or material component. [...]

If the need for a spell’s components has been removed by a special ability, such as the sorcerer’s Subtle Spell feature or the Innate Spellcasting trait possessed by many creatures, the casting of the spell is imperceptible. (XGtE)

This means that casting a spell has absolutely no observable effects outside of the verbal and somatic components as well as the handling of the material components. The effects of the spell only occur when the spell casting has been completed.

This is also supported by Jeremy Crawford:

Do you notice a spell being cast? The answer is based on whether you noticed any of the spellcasting components: V, S, or M.

Fireball as an example

Take Fireball for example. It has (as you mention) verbal, somatic, and material components.

Here is how the book describes the casting of Fireball:

Clad in the silver robes that denote her station, an elf closes her eyes to shut out the distractions of the battlefield and begins her quiet chant. Fingers weaving in front of her, she completes her spell and launches a tiny bead of fire toward the enemy ranks, where it erupts into a conflagration that engulfs the soldiers.

Note the lack of any discernible magic until the spell is completed.

The wizard uses verbal and somatic components ("begins her quiet chant. Fingers weaving in front of her..."), then completes the spell resulting in the effect ("launches a tiny bead of fire toward the enemy ranks").

Counterspell can only be used before a spell is complete

Counterspell says:

You attempt to interrupt a creature in the process of casting a spell.

In the example above, you can only cast Counterspell at the point at which the wizard is muttering and making hand gestures. Once they complete those and "a bright streak flashes from [their] pointing finger" it is too late to Counterspell because the casting has already been completed.

How trying to deceive using any illusion spell would work in practice

Let's say for this example there are two spellcasters: you and your opponent. You can both see and hear each other and you suspect that the next spell you cast your enemy will try to Counterspell. So you decide to try to trick them using Minor Illusion to emulate a Fireball spell.

Case 1: Counterspell right away

You start casting Minor Illusion by making gestures and holding material components. Seeing that you are starting to cast a spell, the enemy right away casts Counterspell and succeeds. Thus your Minor Illusion never completes but you have successfully made them "waste" a Counterspell.

Case 2: Chooses not to Counterspell

You start casting Minor Illusion by making gestures and holding material components. However, your opponent decides to not use Counterspell this time. Maybe they don't even hear or see you start to cast the spell, maybe they are low on slots and decide not to risk it. Either way, they decide not to cast Counterspell.

Thus, your Minor Illusion spell completes. You choose to have it create the effect "a bright streak flashes from your pointing finger". Your opponent sees this and may think for a moment that they have made a mistake and might be tricked into thinking you have cast Fireball. However, at this point, there is no way for them to actually Counterspell you. Your spell has completed and taken its effect and the spellcaster will soon realize that no Fireball is actually coming.

Results

Case 1 actually does exactly what you want it to (make them waste a Counterspell), but it can be done with literally any spell. Unless the wizard has a friend who is good at identifying spells they are not going to be able to identify what you are casting in time to see that the spell is not worth dispelling.

Case 2 does what you think you are attempting to try to make work, but no Counterspell is used. By the time they see your illusionary trick, it is too late to Counterspell it. Thus, defeating the whole purpose of the trick in the first place.

Your characters don't know what spells are being cast

You say:

every time someone announces what spell they are casting like a well placed "Fireball" you or your foe jumps right up screaming "Counter Spell"

However, it is important to note that even if you say "I cast minor illusion", the enemy does not automatically know that. That is why case 1 of my example worked.

Allowing you to know what spell an enemy is casting (or vice-versa) just because the name of the spell is announced at the table is metagaming and not the intended way for spells to function.

What you and your DM have to do is make sure that you are keeping player and character knowledge separate. If you do that, then I think you will find that Counterspell becomes much trickier to use. For one thing, they won't always be able to jump on the big damage spells but might accidentally catch you when you are casting a smaller spell or cantrip.

Most of what you want could not be made with Minor Illusion or Silent Image regardless

Minor Illusion's description limits its usage to either a sound (or series of sounds), or an image of an object (nothing else):

You create a sound or an image of an object within range that lasts for the duration. [...] The image can’t create sound, light, smell, or any other sensory effect.

So it would not be able to create:

a bright glow at the tip of the PC's pointing finger

Or any of these:

Covering yourself in illusory flames to emulate "Investiture of Flame".

Making your hand glow brightly to emulate "Sunbeam".

A levitating disk of light to emulate "Portal"

The description of Silent Image (which can be of "an object, a creature, or some other visible phenomenon") similarly states:

The image is purely visual; it isn't accompanied by sound, smell, or other sensory effects.

Minor Illusion also cannot create any type of movement.