In most Cantonese speakers I know, 廿 is still a colloquial item of vocabulary, replaced with 二十 in usual formal writing; but 廿 remains a very common alternative, for counting as well as enumerating. According to CantoDict, the pronunciation "a" is the most common. This is verified in my experience; the variant with "e" I've not heard this before myself, but I suppose it is possible. (What is written as -ia becoming -ie is something I've heard before).
It has always been perceived as a one-syllable word, with one tone (the 陽去 tone, realised as low and level in standard Cantonese, and noted as 6th tone in Yale and Jyutping). True "polysyllabic" characters do exist in Chinese, but 廿 is not traditionally one of them. The most well known is 囍, but there are more.
In Standard Cantonese, 卅 is saa1, whilst 卌 is sei3 (note 卌 is pronounced the same as 四 when alone). These are somewhat more common in Cantonese than in Mandarin; 卅 is used in colloquial speech referring to the 30th of the lunar month in the traditional calendar, especially in reference to the last day of the year, [Lunar] New Year's Eve, 年卅晚.
The pronunciations 卅呀 saa1-aa6 and 卌呀 sei3-aa6 have been written with 呀 on the end in CantoDict to represent that second lower syllable.
In Mandarin, there are 廿 niàn, 卅 sà and 卌 xì as very literary characters with restricted usage. 廿 is commonly seen in Chinese calendars, but not necessarily commonly pronounced as 廿 niàn, so in Mandarin it could be considered a polysyllabic character. This however is not the traditional method of pronouncing it. MDBG.net gives a few more set expressions with 廿. 卌 is even rarer. But both these characters, along with 廿, have a long pedigree: 卌 is present in the Guangyun.
Among the topolects that use 卅 regularly apart from Cantonese, we also have the Min Nan group and the Wu group (including Shanghainese). 卌 is even rarer, and I don't know if it's used in Wu; but Cantonese and Min Nan attest to its existence.
Note there's also 皕.