An appointment can be for a meeting, but they aren't really interchangeable. A meeting is a group getting together for a specific purpose/discussion. An appointment is a block of time set aside to do something, usually with another person, but it doesn't have to be a meeting, and even when it is, you might say that your appointment is a meeting but you wouldn't say that your meeting is an appointment.
Or if you did, those sentences would mean different things.
"He has forgotten the date" is got refused! But Termin can even take date as translation. At least, that's what Langenscheidt on the Internet is making clear: 1."einer Termin festsetzen" =fix or agree for a date, for doing something with somebody.2."zum nachstmöglichen Termin" =as the earliest possible date, as soon as possible.3."Termin für Manuskriptabgabe" =date for delivery (of manuscript). Thanks to Langenscheidt I am going to report...
No, "Er hat den Termin vergisst" would be like saying "He has forgets the appointment."
You never have two declined forms of a verb in the same sentence, and in your example, hat and vergisst are both declined. When there are two verbs in a sentence, the first auxillary verb (hat, had) is declined, and the second one is either an infinitive or a participle form. Forgotten and vergessen are both past participles, which both also happen to be the same as the infinitive.
As for whether a participle has an -en or -t ending, participles of irregular verbs ("strong" verbs) end in -en, and participles of regular verbs end in -t. ("Weak" verbs.)
A quick test is that, if the vowel changes between the first person singular and the third person singular, then it's an irregular verb.
So "vergesse, vergisst" -> irregular -> hat vergessen.
But "verkaufe, verkauft" -> regular -> hat verkauft.
The infinitive has the same vowel as the first person singular, so many people use that instead: Vergessen, vergisst, hat vergessen Verkaufen, verkauft, hast verkauft.