I understand that the spanish equivalent for habitation certificate is "Licencia de Primera Ocupación" or "Cédula de Habitabilidad" (both are the almost the same in practice).
1--If the above is correct and your solicitor is putting presure for you signing the title deeds, then I think we must consider the following:
Ideally, following the municipalities laws, you shouldn´t live in the house until the habitation certificate has been granted by the Town Hall to the Developer Company. In fact, this certificate means not only that the "Urbanismo" department of the Town Hall finds that the final project that´s been built correspond with the original project ("Licencia de Obras"), but also means that the Town Hall certifies that the house is "habitable". In the practise, many owners sign the title deeds and start living in their house using electricity and water "from the plot", as the Spanish Electricity Company "Sevillana" will not give access to their servicies until the habitation certificate is granted. The same happens with water. This is happening in many cases with normally good results in the end. I wouldn´t do this if I could avoid it, as some inconvenients could arise on a daily basis (energy suply not garanteed).
In many cases this situation won´t be long. Ask your solicitor to find out with the Town Hall the real expected delay for getting the habitation document or the reasons or true risk of not getting it.
2-- All the above changes if what happens is that in the Contract signed with the Developer Company, you agreed that title deeds had to be done NOT when having the habitation certificate BUT when having the architech certificate. Check the Contract, it´s probably reflected there. I understand this is the reason why your solicitor is pressuring you. Check with your solicitor the risk of the Developer Company using their right of cancelling the Contract and getting the property back (giving you or not yout money back).
If this is the situation you´ll have to choose between the risk of loosing the property or live in it while waiting for the habitation certificate.
I am not an expert but check with yout solicitor if the above is similar to your particular situation. Try then to negociate with the Developer.