The warlock's (Pact of the Blade-only) Improved Pact Weapon eldritch invocation (Xanathar's Guide to Everything, p. 57) states that you can use your pact weapon as a spellcasting focus. Using a whip, let's say, would give your weapon a reach of 10 feet.

Using a reach weapon as a spellcasting focus, can you deliver a touch spell through it to a creature 10 feet away from you?

No, a focus does not deliver spells

Page 202 of the Player's Handbook states (emphasis mine):

Material (M)

Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.

If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell. A spellcaster must have a hand free to access these components - or to hold a spellcasting focus - but it can be the same hand that he or she uses to perform somatic components.

Normally you would need one empty hand to hold your focus and do somatic components of the spell. Thus, could not hold a weapon and shield while casting. In effect, the Pact of the Blade feature removes this restriction by making your weapon a focus.

No.

Using something as a spell focus means you no longer need to provide costless material components when casting a spell (Player's Handbook, p. 203, section on material components). There is no provision for delivering touch attacks through your spell focus, so unless a specific rule (e.g., an item or spell) says otherwise, you cannot do that.

RAW says no (or tenuously unclear)

Touch spells have no special rules apart from this bit in Player's Handbook, page 202, under Range:

Some spells can target only a creature (including you) that you touch.

Lacking a proper definition, to touch a target means the same as in natural English: you need to have body contact.

I am aware that it's also idiomatic to "touch someone with something", however, this is not a very natural reading of the rules to me and would empower any caster to acquire a long, thin pole to poke their spell targets with.

Whichever reading you prefer, the fact that the weapon is one's focus doesn't have any impact on using it to deliver touch spells.