Discoloured shower room tiles and cracked grout

Our house was renovated 5 years ago, builders were pretty shoddy. One of our shower rooms is hardly used but recently in the last 2 weeks we have been using it daily multiple times. I noticed some discoloration on tiles and on closer inspection noticed some hairline cracks in some grout near the bottom. We pulled out the Silicon and theres some mould under it in a few areas. We were told all our wetrooms would be tanked but looking back at our old pictures kinda doubt it was done properly. Pressing on the tiles they dont feel loose. Im just wondering if we can try to just dig one some grout where affected to see if I can dry it out and just reseal everything with grout and Silicon the edges and corners? Obviously want easy way out of this ! First and second photo is of the bathroon when they were tiling. Pic 3-5 is of the discoloration and cracks, and last photo is 'tanking' done in another bathroom to give you an idea of their 'work'...
Appreciate your advice!!