So this prompted a thought for me. I've been trying to get my head around the choice of three dialects. As if I need to pick one to focus on to learn. Your answer makes me wonder if that isn't necessary. If I were to mix dialects in my usage, would I be understood? (This makes an assumption that I will ever be able to pronounce anything!!) Of course dialects in English aren't so dramatic. But they are very distinct. Yet if I mix regional dialects in word choice and accent (which I do), I will be understood. Sometimes I do it on purpose, because I enjoy the juxtaposition and have some favorites from both the deep South and from New England.
Who are you worried won't understand you? Anyone who is willing to talk to you in Irish has already indicated that speaking in Irish is a higher priority than absolute clarity - unlike French or Spanish or German or most other languages, you can't speak to a monoglot Irish speaker - all the Irish speakers in Ireland are fluent English speakers too.
Even if you restrict yourself to a minority dialect (and all of the dialects are minority dialects) it's unlikely that you would speak it with the speed and fluency of a native speaker, so you might actually be easier to understand for the majority of Irish speakers who don't speak that specific dialect, but by and large, people who are willing to talk to you in Irish won't usually expect you to have 100% perfect dialect Irish, and if you do have one dialect down pat, it would be expected that you'd be sufficiently experienced to deal with a mix of dialects from other people.
In short, of all the "mistakes" that learners make that would be a barrier to communication, mixing dialect forms is less likely to be a problem than fractured syntax, poor pronunciation and unnatural cadence. By the time you get those issues sorted out, you'll probably have enough exposure to common dialect forms to have developed a degree of consistency.
It's not pronounced "ke vev". The terminal sound is completely different from the initial sound in the second word.
Depending on your browser, you may be able to slow the audio down to half speed by opening this link and right-clicking on the player control:
(It works well in Microsoft Edge, not so well in Firefox).
If you can't use a modern browser, you should still be able to download the link as an MP3 file, and use some utility live VLC to slow it down during playback.
The initial "v" sound in mhéad is made without the tongue, with your lower lip touching your upper teeth.The terminal "d" sound is made with the tongue touching between the alveolar ridge and your teeth, and the lower lip doesn't come into play. The sounds are quite distinct.
It is typically a "softer" sound that the "d" in an English word like "made" - perhaps somewhere between "made" and "lathe".
As far as I know, it's just a dialect difference. The FGB entry for méad suggests that they are interchangeable, but gives examples using only cá. It's sister dictionary at focloir.ie also prefers cá mhéad, with only a couple of examples of cé mhéad. potafocal.com, on the other hand, prefers cé mhéad over cá mhéad.
Duolingo has an exercise for cá mhéad? which it translates as "how much?", but both cé mhéad? and cá mhéad? can be used for either "how much?" or "how many?".