Don't forget #3: Having colonies of any sort!
The Welsh established colonies in Patagonia. Patagonians still speak Welsh to this day (one of the three dialects the ailing language has).
Wales would get the Scotts Irish, and French to attack Britain and the Welsh people would immigrate to these countries spreading there nationality
They tried that on numerous occasions. The most successful attempt happened under Owain Glyndwr, who actually successfully managed to land French troops in England. After some initial successes, he too was thwarted.
The problem is that 1.) Wales has never had the economy or the population to mount any kind of well organized campaign on England. 2.) Their style of combat (mostly guerilla style warfare involving hit and run tactics with longbowmen, and waiting until the English were forced to retreat due to the onset of weather) were not conducive to any sort of offensive campaign.
No it could not. Forgetting that, again, Wales has never had the economy or the manpower to be anything other than a backwater nation which occasionally had some successes against English incursions, Wales had a number of problems that it could not rectify. The largest (IMO) is the fact that the Welsh were a heavily splintered people. Welsh kingdoms operated on an even distribution inheriting system. This means that when a King dies, everything is divided evenly amongst his sons. So even when a great military commander (such as, say, Owain mawr) manages to successfully unify and turn his kingdom into a respectable nation, when he dies that kingdom gets broken up among his several (or at times numerous) sons (also note, that in Wales a bastard son was considered just as legitimate a son to your lawfully wedded wife), who naturally immediately set about fighting each other for control of the nation.
Now, this changed under Llywelyn fawr, who decided to put his entire kingdom under the control of his son Dafydd, who followed likewise with Llywelyn the last, but by then the English had learned how to defeat the Welsh (that is, capturing Ynys Mon, and then just building a crapton of castles all over the place to facilitate the influx of supplies, and prevent raids preventing reinforcements from coming in, not to mention allowing them to stay in Wales for multiple years).
Another big problem was the lack of any sort of farmland in the country. The Welsh are primarily shepherds, with the only considerable amount of farmland in the country being located on Ynys Mon (Anglesey), which was, as I said, easily captured by the English (it fell out of Welsh control entirely by the time Edward I got his hands on it).
So, if you can rectify the fact that Wales's population was not only extremely small, but extremely splintered among severally equally powerful principalities, its economy nowhere near powerful enough to mount any sort of serious invasion of foreign lands, an inheritance system that was not really conducive to consolidation of conquered lands over the course of several generations, a military system that didn't work well offensively, and the fact that it had trouble convincing allies to take it seriously enough to help them when they asked for it, then yes, it could happen. Until then, not really.