1.) Which keyboards from Yamaha can I consider for making youtube covers, as well as live stage performances ?
I'd think you can consider arrangers like Genos, Tyros, PSR-S, or other high-level PSR models (such as PSR-A). And you can also consider synths, as well as digital pianos, depending on what you're looking for as far as features, portability, etc.
2.) Should it be an arranger or a synthesizer, the one that provides me with a professional level sound quality (output) of different instrument voices ?
That's up to you. If you make heavy use of styles while performing, then you'll probably want an arranger. But if you don't use styles and want something that gives you more control over the sounds while you're playing live, then you'll probably want a synth.
3.) Also, what are the hassles or gaps that would be created when I move from an arranger to a synthesizer like MX-61 ? Do S series keyboards provide better sound output than synthesizers like MX-61, or is it vice versa ?
The biggest gap when going from an arranger to a synth will be the lack of styles or auto accompaniments. I'd think the quality of the sound would depend more on the model than on whether it's an arranger or a synth.
4.) Is MX-61 an expandable synthesizer or not ? Can I add new patches/loops to it, or do I need to increase my budget to get an expandable synthesizer from Yamaha ? Please mention the model names.
No, the MX line is not expandable in the sense of being able to load new wave samples to it as you can do with the MOXF and MODX lines. However, you can still load or create new voices from the existing elements (wave samples), by mixing different ones together or by changing their sound parameters (similar to how you can customize voices on the PSR-E453, but to a much greater extent). Or you can simply customize the preset voices, or layer them together however you wish-- up to 16 at once, called a "performance." There are some free performance packs available, plus software for creating voices and performances. (The tiny LCD screen and function menu make it difficult to do those things on the synth itself, so the software is extremely useful because it lets you do it on a computer.)
Note that the MX can also use a free iPad app from Yamaha which lets you create and play FM synth voices, and you can certainly play other iPad soft synths with it as well. (For that matter, you can do this with your PSR-E453 as well.)
If the MX synths are not flexible enough for you, you might consider one of the MODX synths, or perhaps an older MOXF, MOX, or MO synth. Of course, if cost is not an issue, then go for the Montage or Motif!
Frankly, you first need to decide whether you'd be happiest with an arranger so you can play auto accompaniments. But another possibility might be to get one of each-- one of the PSR-S arrangers, plus the MX49 Mk2 synth (because it's the lowest-priced one yet has all the same features of the MX61 and MX88).
5.) I'm from India, here we have Indian Classical music and a wide variety of Indian percussion/stringed instruments, with their roots diving deep into the existing music industry here. So are there some good options in Yamaha to fulfill these, or do I need to move out to some other brands like Roland, Korg etc ?
Have you looked at the higher PSR-A models, such as the PSR-A3000? I don't think you'd be happy with the lower PSR-A models, such as the PSR-A350.
