There are many ways to find accommodation in the towns and tourist destinations on the coast: major online booking services and local websites. I am not going to cover them here.
There is a network of mountain huts. Those at Platak are open all year round and provide beds, food and bars. Others are open at weekends except in winter. They will have a room accessible in winter with bunks and a wood stove. Smaller huts are unmanned and basic.
There are plenty of campsites on the coast. Most are large with entertainments and pools, and a price tag to match. There are smaller, cheaper campsite if that is more your thing, most are listed here.
Wild camping is illegal in Croatia. Outside of the national parks you are unlikely to have any problems, however there are plenty of huts so there is rarely much need to carry a tent.
Similarly camping on the beaches is also illegal, but given the numerous isolated and beautiful beaches it is very, very tempting - but please e discreet.
Mobile phones work virtually everywhere in Croatia. Close to the Slovenian border your phone might choose a Slovenian network but with the abolition of roaming charges within the EU this isn't a problem for most visitors.
If you want to buy a local SIM card then my recommendation is Tomato SIM cards can be bought from any VIP phone shops. Monthly package without a contract can be bought at remarkably low prices. Be aware that the Croatian government apply an extra 10% tax on phone credit, so 20 kuna credit will cost 22 kuna.
The mountain rescue service (GSS) and Coastguard can be reached on the usual EU number: 112
It is certainly possible to rely exclusively on mobile phones for navigation. Personally I feel safer, particularly when hiking in the high mountains to carry a paper map. SMAND produce hiking maps of almost all mountain areas and are available from most larger book shops in Rijeka and other large towns for about 70 kuna but the full range is not always available. Iglu Sport on Vukovarska 67 in Rijeka usually has a better range and charge only 50 kuna so worth a visit if you want to buy several maps.
Bears - there are bears living throughout the forest of Croatia. However are unlikely to be lucky/unlucky enough to see one. I've seen two in eight years, one crossing the main road in Kostrena just south of Rijeka. They are brown bears, if you want to learn what to do if you meet one then check on youtube of similar. I don't want to be sued for giving the wrong advice.
Snakes - there are several species of snake in the region, some are poisonous. Every year people are bitten but usually during the wild asparagus season when people surprise them by searching through the undergrowth.
Water - getting water is a real issue in the mountains and a far greater threat than bears or snakes. The landscape is limestone and rain water quickly disappears into the ground. I always carry at least two litres in summer and have run short a couple of times. Most of the smaller huts are placed by water sources or have rainwater collection systems but they are not always maintained. Although I've never had any problem it is probably worth carrying water purification tablets on an multiday hike.