I've no idea when the change that you ask about occurred. The definition below from the OED has not been updated since 1982, indicating that back then it did not consider the schwa to be exclusively reserved for unstressed syllables.
A Dictionary of Psychology (4th edition, Oxford U Press) says:
The neutral and central mid vowel...that occurs in the words the and fern, at the beginning of about, and at the end of sofa, and the symbol in the International Phonetic Alphabet (IPA) that represents it, namely an inverted e. Statistically, it is the most frequently occurring English vowel (over ten per cent of all vowel sounds), yet it has no corresponding single letter in the standard alphabet. See also central vowel, formant, mid vowel. [From Hebrew shewa a mark indicating the absence of a vowel sound][my emphasis in bold]
The Oxford English Dictionary (OED) defines schwa as
The central vowel sound /ə/ , typically occurring in weakly stressed syllables, as in the final syllable of ‘sofa’ and the first syllable of ‘along’; = sheva n. 2. Occas., the symbol of an inverted ‘e’ used to represent this sound.
It gives a schwa in the pronunciation of such single syllable words as sir, purr, bird and the stressed syllable of birdbath.
The aptly named Concise Oxford Dictionary of Linguistics says
The mid-central vowel of e.g. the second syllable of matter: in phonetic notation [ə] ([matə]). Also spelled ‘shwa’.
Fowler’s Dictionary of Modern English Usage (4 ed.):
/ʃwɑː/ (shwah). In the phrase ‘a moment ago’ in unemphatic BrE speech, the two unstressed vowels in italics are pronounced identically. The technical name for this sound is schwa, and its symbol is /ə/ in the International Phonetic Alphabet. Not only letter a can be pronounced /ə/ : the italicized letters in the following show it represented by other written vowels: number, the, obey, commit, success, picture. The fact that letters a, e, o, and u can all be pronounced as a schwa explains many widespread spelling mistakes, such as *relevent.
I'll add that's it's too bad the sound of schwa is not also schwa.
Generally, double consonants are not pronounced distinctly in English, unless they are part of different syllables and the emphasis is on the second syllable.
A word like dissatisfied is formed by adding a prefix dis- to the word satisfied. It starts off with two s in separate syllables, and can be pronounced like that- one at the end of the first syllable, one at the end of the second syllable.
A word like irregular is formed by adding a prefix in- to the word regular. The n-r combination is difficult to say, so we replace the n by another r. The same thing happens with the letter l, so in + licit becomes illicit. According to Cambridge Dictionary, the first l is not pronounced, likewise with in + modest. Note that this conversion only happens with word that passed through medieval latin: more modern words like inroad (1540), inlay (16th C) and inline (1913) are unaffected.
The same kind of conversion happens in arabic for sun letters (il+r -> irr). In arabic double consonants are always clearly pronounced, and this applies to sun letter conversions too.
In a non-rhotic dialects there is an identifiable reaason for not pronouncing the first r, because in non-rhotic dialects (England english, for example) an r followed by a consonant is not pronounced.
In rhotic dialects such as US english, the pronunciation of the n-become-r is, according to Merriam-Webster, optional.
I am a native of England (non-rhotic) and I do not pronounce it as a double r. I can and do double the r when speaking arabic, so I do understand the difference. Other natives of England do not pronounce the double r. If I heard somebody pronounce it with a double r, I would assume that they were foreign. I believe that I have heard natives of Scotland (rhotic) pronouncing it with a double r. I cannot comment on US english.
Here are recordings of me saying irregular and erectile:
And here I say irregular again, pronouncing the two r's separately.
Best Answer
The KIT vowel, /ɪ/, which we find in the word sit, and the FLEECE vowel, /i/, which we find in seat have different pronunciations in Southern Standard British English and General American. In Gen Am they are said with the jaw slightly more open than in British English. So a Gen Am /ɪ/ may sound more /e/-like to someone used to British English.
In the Western United states, the KIT vowel changes when it occurs before velar consonants /k, g/ or /ŋ/. We say that this new sound is an allophone of the KIT vowel for these speakers. When people who speak this variety of English have a KIT vowel before a velar consonant it becomes much more like a FLEECE vowel. It is longer and more close.
Because of this when these speakers say, for example the word sing, it will sound a lot like the word seeing to somebody who does not have the same accent. These speakers also have a different vowel in words like bag. Here again we have a velar consonant, /g/. These speakers will use a vowel that sounds like /eɪ/ to other listeners. For people listening, the word bag may seem to rhyme with the word plague.
These different possibilities for the vowels which occur in words like king and kin could be why the Original Poster finds these types of word confusing.
This article here talks about Western US vowels before velar consonants