a possible solution

There is a possible not enought explored solution. And i think that is the best way. I'll try to explain in my awful english:

By one side, instead of voting an Independence Declaration without capability to make it effective, what should have had been voted is the abolition of the Nueva Planta Decree. The Nueva Planta Decrees are still a source of law (https://www.diarilaveu.com/noticia/21070/el-tc-es-remet-al-decret-de-nova-planta-per-justificar-leliminacio-del-dret-civil-valencia#.WYl8YJrC1U-.twitter ) 
Catalan Parliament would have had the right to vote that. And that would have changed everything. 
Those decrees are still alive and they are there because of the "Right of conquest", which is their source of legitimacy. And that source, nowadays, is not appropiate nor acceptable. 
Thats historic legitimacy. 
Our demonstrations on 11 Sep. are because of those decrees and the abolition of the catalan constitutions. 
The landscape that would open that would be wide enough as to let to open an update period of the ancient constitutions to the international rights and charters and the democratic principles. Which catalanist party would have opposed to the abolition of the Nueva Planta Decrees ? none. Just Pp and Ciudadanos. 
The landscape opened by that, linked to the real , natural and ancient right of the catalans, much older than the current constitution, would have been a nearly full independence. The step from there to the independent republic, as Pau Claris declared on the xvii th century would have been much shorter. 
And the catalan constitucions would have permited to have an army, the ancient Miquelets (https://ca.wikipedia.org/wiki/Miquelet ). 
I should remember here that the Barcelonian County title is still existing, and is vacant nowadays. And that the catalan parliament, under the catalan constitutions, could have given that title to someone , nominally some kind of chief of state but obligated to obey the catalan laws, to take it. 

If we have the Generalitat (from 1359) and the Mossos (from 1720 aprox.), and Gibraltar or Luxembourg existance are examples of the vitality of those pacts and decrees, then recalling and updating our legitimate laws would have been easier, and with wider support, than our current situation. 

And, by the other side, the key point, before the Declaration of the Republic, would have been to nominate as to count of Barcelona to someone like the Bishop of Urgell, catalan and already a Chief of State as co-princep d'Andorra. that means that he already has a chair in the UN. 

Other possibilities could have been explored, as they were in the past of our history. 

here, wrote in catalan, a little longer explanation of the issue https://pequod-tng.blogspot.com.es/2017/11/hi-ha-un-elefant-al-mig-de-lhabitacio.html , in the second part of that entry. begins where it says "una possible sortida".

the speech of Pau Claris, in catalan, in the catalan parliament (junta de braços) answering to the speech of the royalist Pau Duran, according to Francisco Manuel de Melo, can be found here:
https://www.llibertat.cat/2014/06/discurs-de-pau-claris-on-es-va-decidir-la-resistencia-contra-els-exercits-d-ocupacio-de-felip-iv-26697 and here http://www.lasega1640.cat/?tag=pau-claris 
"Els homes van fer els reis i no pas els reis als homes" : "were the men who created the kings and not the kings to the men"


https://en.wikipedia.org/wiki/Catalan_Republic_(1641)

http://www.devolucio.cat/constitucions/01.html

https://www.parlament.cat/web/documentacio/cataleg/fitxes/index.html?p_pagina=2&p_fw=16663218