edit: I missed the 1991-1995 part. TL;DR is there was no crossing of any kind between Croatia and RSK.

1. Most if not all paved roads had UN checkpoint between Serb and Croats by 1994, you could see remains for years after the war. There were probably other checkpoints on strategically important points.

2. You couldn't normally cross to the other side before 1995. By the time you could cross to the other side, RSK was finished and reintegration has started. I crossed weeks after end of the war, there was Croatian and Serb checkpoint and UN (Jordanian) checkpoint in between. Both were manned by police due to demilitarization and only ID cards were required.

There was never anything resembling proper border. Until late 1995 two sides were in a state of war, even though there were long periods of no shots fired. Also establishing anything resembling border would give legitimacy to RSK.

All checkpoints were gone by the end of peaceful reintegration in Slavonia. No checkpoints at all in liberated parts of the country, except military checkpoints in first months after armed operations.

source: lived in close proximity to the front lines and UN checkpoint in Vinkovci, eastern Slavonia. Crossed at first opportunity after the war to meet family at the other side, and later for cheap shopping in Vukovar before their prices cought up with full reintegration.

As for the other questions, I am less competent. I did cross to Slovenia in autumn 1991 in a train and there was passport control, albeit I was in a train with 1000 other refugees so it mustn't have been very thorough check. Don't remember much of that ride other than lacking a place to sit in a train. I'm not sure where to even find out when exactly border was established other than go and look for digitalized news articles from 1991 and that's a huge endavor. However, a full border was established but local crossings for citizens of villages on the border were quite open as closing them down was a problem when Slovenia entered EU. But Croatia had ID card checks only with all neighbors for quite some time, I don't even remember when was the last time I had to have a passport, and Slovenia was probably first country you could visit only with ID card.

First time I crossed to Bosnia was around 1997, over ferry which replaced destroyed bridge at Županja-Orašje crossing. Orašje was in Croat control but there was a full, normal border with customs and all. But that's a couple of years after war. I really don't know the situation during the war in the south by my best guess was that there was no border crossings, just polce checkpoint with ID card check, if that. Herceg-Bosna was de facto part of Croatia and in '94 and '95 there wasn't even much effort to hide it.

1. No there were no unofficial border crossings between RSK and Croatia. Except of border crossings which were under control of UN, but even in those you could not just walk by and say: I want to cross. It was only allowed for selected few UN officialsand humanitarian organizations and under some extraordinary circumstances.
2. It was a frontline. Not all parts were thoroughly covered, you could find many gaps to walk by but then you were under risk to get shot or to step on mine. Both sides had patrols covering the area even when large swaths of territory were buffer under so called Un neutral zone.
3. Forgive me but i do not have exact date but border crossings were established soon after the end of Slovenian war. They were manned at first by Yugoslavian army but then Croatians took over. But as my other compatriot said, border was often porose due to huge numbers of refugees, but nevertheless Slovenia keept an eye on border due to the proximity of war.
4. They did in fact exist. Due to the smuggling and desertions, there were checkpoints so that nobody could just walk in or walk out.

Remind me later and i will give you sources, i need some times to find relevant things it was quite some time since i read those things.