I would agree with what BangBangTequila said: to become a saint, you must win the support of the masses. Anything is possible, but you must be quick, determined and have a clarity of purpose.
The first step - since you're on a recently rediscovered shrine world - is to secure your position. I would suggest playing out three cards:
1.1. Address the low-born masses through demagogues (resurrected Adepta Sororitas and some other witnesses of the miracle would make ideal candidates). Let them tell the story of how a great man - Cromwell-blah-blah-blah - fought valiantly and died in His Name, and how one of His ancient Saints descended upon this world, taking Cromwell's broken shell as his own to crush His enemies.
1.2. Dig out the prophecies of this world doom (every shrine world has officially-supressed tales of Judgement Hour and how the corruption of its clergy will come to light and be judged; if this one has no such thing, invent it), and reenact some of them on the least pious ecclesiarchs. Remember, you're fighting for the good of all Mankind here; their deaths are a necessary sacrifice.
1.3. Depending on whether you inquisitor supports you or not, use him. If he is an ally, ask him to silence the most influental enemies in immediate vicinity. If he is an enemy, lure him into executing the said enemies for perceived heresy. It won't be easy, but it must be done if you're to leave the planet alive.
The second step would be to make contact with sympathizing factions within the Ecclesiarchy. Please note these won't be the high-ranking priests, for they are in general content with their place in the organization and won't take kindly to any prospect of religious upheaval.
2.1. So, approach the less-established clergy; depending on your mindset these could be Redemptionists, Conciliators, Temple Tendency… in short, some sub-faction with a burning desire to have 'their own' Saint. If you're really careful, approach several such sub-factions, each time presenting a different aspect of your Sainthood. Ie, you can be Cromwell The Righteous for the Redemptionists and Cromwell The Poor fo the Temple Tendency. Whoever you choose, the goal at this stage is to establish a 'foothold' within the ranks of Ecclesiarchy, a cadre of clerics who will support your official claim later.
The third step is the most difficult one: you must make your enemies to show their hand and then disgrace them.
3.1. By now the news of religious upheaval on the original shrine world would probably reach the ears of Ecclesiarchy. If you did everything right up to this point, the crisis would be too serious to ignore, yet not serious enough for Ecclesiarchy to call upon the aid of Inquisition. Indeed, the Ecclesiarchy is notorious for its desire to keep their skeletons for themselves, so to speak, so at this point it is unlikely they will call for help.
3.2. Most likely, the punitive force would include a cadre of high-ranking ecclesiarchs and some Adepta Sororitas regiments. You must break them in the subsequent War of Faith, but inflicting as few casualties on the Adepta Sororitas as possible. In the end, they must be humiliated and beaten, but not be slaughtered.
3.3. Appear before the canoness openly, escorted by your own Sororitas warrior-sisters (you had at least two) and plead your case. Explain how you - the spirit of a long-dead Saint - returned to this world (answering a prayer of a dying mr.Cromwell) and how you found the world steeped in corruption, and set upon cleansing it. Tell her how reluctant you were to fight (apparently) the misguided Adepta Sororitas, and how much it resembles the Times of Blood, when Brides of the Emperor mistakenly fought for crazed Vandire before finally seeing the light of truth and siding with Sebastian Thor…
3.4. Make the whole meeting secretly broadcast to you followers; a combination of pict-flies and good vox-caster relays would do the trick.
3.5. Most likely, the canoness will declare you a heretic and kill you where you stand. In this case, be prepared to 'resurrect' yourself once more. Do not attack the canoness yet, but tell her how He forgives her lack of vision and gives her one more chance to see the light. Recite once again the tragic tale of Vandire's last days. Probably, you'll be shot to pieces or cut down with the sword once more, so this time activate the Rosarius.
3.6. As the shot / strike leaves you unscathed, call upon the will of the Emperor to help the lost soul (aka 'canoness') to see the light… or to strike her down if she is irredeemably corrupt. She will probably attack you again in outrage because of your last remark…
3.7. Seize this opportunity to kill her; Disease spell wold work best, making the wench retch blood and writhe in obvious pain, as the 'Emperor's Light' abandons her. Her subsequent turning into a disease-ridden monstrosity would bring the corruption of your enemies to light, and galvanize your case.
3.8. Offer the shocked audience one last chance to abandon their corrupt ways and embrace the Light of the Him On Earth. If they bow down to you, you've won. If they don't, give your followers an order to attack, and this time don't refrain from the slaughter; all non-believers must be eliminated.
3.9. In the aftermath (whatever it is), contract your allies within the Ecclesiarchy and ask them to start the process, then cease to be for a time. If you ever feel the process stalls, resurface and repeat steps 1.1-1.3 and 3.1-3.8 on some other world, but generally make yourself scarce. The process would take years (if not decades), but if you remain true and determined, you'll be a Saint before long.
Failing that, a Mark of Tzeentch is also a worthy reward