We have the exact same opinion! If Charles X gave Paris another whiff of Grapeshot the Restoration would have likely continued. Though it wouldn't be necessary to raise a new force; he could have easily used the Army and the Royal Guard (25,000 or so). I remember that the units in Paris did fight during the Trois Glorieuses but lack of supplies, orders and numbers led to them slipping away or dissolving. A major issue was the invasion of Algiers; a good deal of the army was therefore absent when the revolution took place, so perhaps if the more elite loyal regiments are kept in France while the disloyal ones are sent to Algeria the Bourbons might have enough men to put the Parisians in their place. However this is a bit off topic to what I'm proposing. You are right about this part though; Paris was a beast to keep happy and it almost wasn't worth it.
I am afraid you are overestimating the loyalty of the Army towards the Bourbon. The NCO and junior officers were, in their majority, former soldiers of Napoléon's army, which memory, as time went on, was more and more leaning towards the "golden legend". During the 1830 Revolution, at least two full regiments (5th and 53th of Foot) went on the Revolution's side, two others (15th and 56th) were sent to Versailles by Marmont because they were on the verge of doing so, and some sub-units of the 50th rebelled as well. Only the Royal Guard, especially the Swiss units, remained loyal to Charles X.
Even if Marmont had managed to crush the Revolution by sending for artillery and blowing up borough after borough, how long can the regime hold ? Their own élites did not support them any longer, the Army loyalty hanged by a thread, and you can imagine the love of Paris people after such a repression.
Not at all the way I'm interested in. Plus the Orléans were WAY to closely associated with the revolutionaries to be acceptable monarchs for the Royalists. If anything Louis XVIII would likely adopt one of the Spanish Borbóns, like Infante Carlos or Infante Francisco, as his heir. With the stipulation that they forever renounce any rights to Spain and any kind of Franco-Spanish union. Hm, that could actually be a cool idea. Also Louis Philippe would likely be Philippe VIII; that was the regnal name proposed OTL when it was debated whether or not he would be traditional King of France or popular King of the French.
No, he could not. His hands are tied with his own legitimacy : the fundamental laws. The throne can only go to two persons, depends on which system of succession you chose : Louis-Philippe of Orléans or King Ferdinand IV. Louis XVIII cannot adopt (adoption did not exist under Ancien Régime law and was severely restricted under Napoléon's Code civil). He must either choose an inept absolutist King who needed a French expedition to keep his own throne, one which succession in France would likely create a major international crisis, or an intelligent if unfaithful Frenchman with extensive political influence and national recognition. Louis XVIII was an intelligent man : who do you think he would choose ?
Sure, the Ultras would not like Philippe VII (not VIII), but Louis XVIII was not an Ultra himself and, ITTL, Decazes would continue to stay PM (no Berry, so no Berry assassination). Decazes, Louis' favorite, went along with Louis-Philippe perfectly well after 1830. He would probably advise for an Orleans succession in the 1820' and Louis would probably follow.