According to Duo, when writing the definite form of a noun: for common nouns ending in consonants, the -en suffix is used; unless the noun ends in -ar, -el, -en, and -er.
In this case is it always the seasoning? As in there is another word for an actual pepper?
A bell pepper = en paprika
The spicy small red/green/orange ones = en chili / en chilifrukt
Yes, if you refer to 'peppar' in Swedish it is always spices like vitpeppar, svartpeppar and so on. The red paprika type fruit is usually called 'chilifrukt' even though its real/botanical name includes 'peppar'.
Try using the website forvo- natives pronounce words and it's got pretty much every language. It's helped me a ton and I've been using it in my swedish course on memrise.
This happens a lot with vowels followed by an r or similar consonant where the r is assimilated by the vowel. The vowel structure of the noun also lends itself to the dropping of the "e" in the direct article so over time the pronunciation of the r became less pronounced. It makes sense when you think about how many language favor two or three syllables and the first thing to go in accommodating the simpler syllable structure are the vowels, especially vowels like "a" and "e". Most irregularities of this nature are generally due to the tendency to simplify speech.
It sounds to me like the second accent, though. So, the word stress is pretty much on both syllables.
I believe (and Wiktionary agrees with me) it's an uncountable verb, so there is no plural form.
I agree, but I think I'd be fine with en peppar, actually - then using två pepparsorter or similar for the plural.
It seems like the article en doesn't really fit for pepper. It's also riset (-> ett ris) and saltet (ett salt), so it's strange that you say "pepparn" which indicates the base form must be "en peppar"