Learn English – How “lie” in present simple becomes “lying” in present continuous

In the first clause, "eBay ..... now [spend] $20m a year analysing suspicious sales" the verbs spend is wrongly conjugated. It should be spends because eBay is a singular noun in this sense.

As you once believed, the adverb now can be used regardless of the verb being stative or dynamic. You were right.

eBay now spends $20m a year analysing suspicious sales.

This is the correct present simple form.

eBay is now spending $20m a year analysing suspicious sales.

This is the correct present continuous form.

Your initial examples are not as unambiguous as you think, and that is probably leading to later confusion. The continuous effect is not as baked in as you make it out to be.

"I'm having dinner at 2:00" actually sounds like dinner is scheduled to start at 2:00.

"I was having dinner at 2:00" could it was ongoing in the past at 2:00, or the 'was' could be subjunctive introducing doubt, meaning plans were changed ("I was having dinner at 2:00, but I missed the bus so we ate at 3:00"). Or even just that plans have been cast into doubt (eg "I was going to Toshi Station to pick up some power converters" -L. Skywalker)

"I will be having dinner at 2:00" could be grammatically be either ongoing or a scheduled start.

Your simple tense examples mostly work, although "I have my dinner at 2:00" sounds like it means "I have dinner at 2:00 every day" more normal usage for a specific upcoming event is "I will have dinner at 2:00"

Anyway, the main point is that the 'ongoing nature' isn't as firm a rule as you are looking for. So that's why there's nothing broken about "I'm having dinner at 2:00 tomorrow" with 2:00 being the start time.