How long a particular string setup lasts is strangely a personal thing. There's no one single golden rule or formula for determining a string's lifespan, except that ordinary poly strings don't break easily (but they don't hold up tension very well because they don't stretch like other strings do), softer strings like a multi will breaks relatively easier (but holds tension better than a poly), and a natural gut string does not last too long (but even so, I've heard some people playing the same natural gut for weeks!).
To answer your question, you should experiment by yourself. I, for instance, would never know how long my strings last because I'd have them cut long before they break; I'm a bit too sensitive to the tension and playability change. This may be your case, and you will never need to know when your string will break.
If you really want a figure, I can give you a rough idea. I play with hybrid setups like yours (in fact I've tried your exact setup twice). The multi (on the crosses in my case) starts to fray quite seriously after about three weeks of play (2 hours a day, about 2-3 days a week). I guess it would break in another 1-2 months time. But I'm sure you'll notice the playability drops drastically long before it breaks.
I know XCel very well, and I know it's an extremely soft string. Right now, I'm trying a multi called ISOSPEED Control on my crosses in my hybrid setup. When I was stringing it, it felt so much tougher, so much stronger than XCel. But when I played with it today, it felt as good as XCel; if anything it was better. But I don't know, it was only the first day I tried this multi, but judging by its tough feel when I was stringing it, I think it should last longer than XCel. You could keep the Babolat Pro Hurricane Tour as your poly choice, and then try this ISOSPEED string, or the Babolat XCel Power. I heard this new "Power" version is a tougher version of XCel Premium, I've never tried it though; it's too expensive to try.
So you've decided to do an all-poly setup then? I checked, ISOSPEED Baseline Long Life is a poly string, and it's only available in reel (long enough for 16.5 rackets), are you sure you want to do this?
A poly-poly hybrid is not something you must not do, but it's something people rarely do, because it doesn't quite make sense. This reason to hybrid poly with a multi is to soft the hard feel of the poly. Although hybrid stringing can be as simple as putting the exact same string and varying only the tension, or the gauge, I don't think you'll notice the difference, especially when you're stringing at that high tension.
I would suggest stringing a one-piece Pro Hurricane Tour. Or try the hybrid I mentioned last time (I'm actually playing now with a KSix-One 95 16x18 with Signum Pro Poly-Plasma 16L with ISOSPEED Control at 50/52. Economic and super performance.)
But there's one more thing. How may pounds do you string when you have a one-piece multi string setup? Because for most poly strings, it's recommended to reduce the tension by 10%, or 4-6 lbs (which are effectively the same thing) than your usual multi setup. I play my one-piece multi at around 55-57 lbs, so I'm stringing my poly in my hybrid at 50. You may wanna lower your tension a little bit when doing a one-piece poly string job, unless you're quite certain what you're doing.
Q1. That really depends on you. If you find your current setting (the one which has just broken) comfortable enough to play with, then you may wanna stick with 61lbs. But I would suggest at least lower 2lbs, because the XCel in your last setting really soften things up and may have confused your judgement. The main question, however, is how many pounds you normally string (for the same or a similar racket) with a one-piece multi.
Q2, 3. Pro Hurricane Tour + ISOSPEED Control should play pretty much the same as crossing with XCel, and should last longer because as I told you before, it felt really tough when being strung, but really soft when being played. Pro Hurricane Tour one-piece job should feel pretty similar to Pro Hurricane Tour with Baseline Long Life. Again, as I told you before, Baseline Long Life is a very simple poly string. People rarely do a poly-poly hybrid, because I would imagine it plays similar to a one-piece poly string job. Both Pro Hurricane one-piece, and Hurricane crossing Baseline Long Life (which I don't recommend) should last surprisingly long; probably takes more than a year to break. But, mind you, the tension maintenance of poly strings is usually poor, because poly doesn't stretch like multi and will snap back into the original position very soon. Today it is 61 lbs, next month could be 58,59.
The best thing to do is to actually get out there and try. Listening to people's opinion about string setups usually doesn't help much with your own setups, because everybody plays and feels differently.